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[Xen-devel] [RFC v2] xSplice design


During the Xen Hacka^H^H^H^HProject Summit? we chatted about live-patching
the hypervisor. We sketched out how it could be done, and brainstormed
some of the problems.

I took that and wrote an design - which is very much RFC. The design is
laid out in two sections - the format of the ELF payload - and then the
hypercalls to act on it.

Hypercall preemption has caused a couple of XSAs so I've baked the need
for that in the design so we hopefully won't have an XSA for this code.

There are two big *TODO* in the design which I had hoped to get done
before sending this out - however I am going on vacation for two weeks
so I figured it would be better to send this off for folks to mull now
then to have it languish.

Please feel free to add more folks on the CC list.


# xSplice Design v1 (EXTERNAL RFC v2)

## Rationale

A mechanism is required to binarily patch the running hypervisor with new
opcodes that have come about due to primarily security updates.

This document describes the design of the API that would allow us to
upload to the hypervisor binary patches.

## Glossary

 * splice - patch in the binary code with new opcodes
 * trampoline - a jump to a new instruction.
 * payload - telemetries of the old code along with binary blob of the new
   function (if needed).
 * reloc - telemetries contained in the payload to construct proper trampoline.

## Multiple ways to patch

The mechanism needs to be flexible to patch the hypervisor in multiple ways
and be as simple as possible. The compiled code is contiguous in memory with
no gaps - so we have no luxury of 'moving' existing code and must either
insert a trampoline to the new code to be executed - or only modify in-place
the code if there is sufficient space. The placement of new code has to be done
by hypervisor and the virtual address for the new code is allocated dynamically.
This implies that the hypervisor must compute the new offsets when splicing
in the new trampoline code. Where the trampoline is added (inside
the function we are patching or just the callers?) is also important.

To lessen the amount of code in hypervisor, the consumer of the API
is responsible for identifying which mechanism to employ and how many locations
to patch. Combinations of modifying in-place code, adding trampoline, etc
has to be supported. The API should allow read/write any memory within
the hypervisor virtual address space.

We must also have a mechanism to query what has been applied and a mechanism
to revert it if needed.

We must also have a mechanism to: provide an copy of the old code - so that
the hypervisor can verify it against the code in memory; the new code;
the symbol name of the function to be patched; or offset from the symbol;
or virtual address.

The complications that this design will encounter are explained later
in this document.

## Patching code

The first mechanism to patch that comes in mind is in-place replacement.
That is replace the affected code with new code. Unfortunately the x86
ISA is variable size which places limits on how much space we have available
to replace the instructions.

The second mechanism is by replacing the call or jump to the
old function with the address of the new function.

A third mechanism is to add a jump to the new function at the
start of the old function.

### Example of trampoline and in-place splicing

As example we will assume the hypervisor does not have XSA-132 (see
*domctl/sysctl: don't leak hypervisor stack to toolstacks*
4ff3449f0e9d175ceb9551d3f2aecb59273f639d) and we would like to binary patch
the hypervisor with it. The original code looks as so:

   48 89 e0                  mov    %rsp,%rax  
   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  

while the new patched hypervisor would be:

   48 c7 45 b8 00 00 00 00   movq   $0x0,-0x48(%rbp)  
   48 c7 45 c0 00 00 00 00   movq   $0x0,-0x40(%rbp)  
   48 c7 45 c8 00 00 00 00   movq   $0x0,-0x38(%rbp)  
   48 89 e0                  mov    %rsp,%rax  
   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  

This is inside the arch_do_domctl. This new change adds 21 extra
bytes of code which alters all the offsets inside the function. To alter
these offsets and add the extra 21 bytes of code we might not have enough
space in .text to squeze this in.

As such we could simplify this problem by only patching the site
which calls arch_do_domctl:

 e8 4b b1 05 00          callq  ffff82d08015fbb9 <arch_do_domctl>  

with a new address for where the new `arch_do_domctl` would be (this
area would be allocated dynamically).

Astute readers will wonder what we need to do if we were to patch `do_domctl`
- which is not called directly by hypervisor but on behalf of the guests via
the `compat_hypercall_table` and `hypercall_table`.
Patching the offset in `hypercall_table` for `do_domctl:
(ffff82d080103079 <do_domctl>:)

 ffff82d08024d490:   79 30  
 ffff82d08024d492:   10 80 d0 82 ff ff   

with the new address where the new `do_domctl` is possible. The other
place where it is used is in `hvm_hypercall64_table` which would need
to be patched in a similar way. This would require an in-place splicing
of the new virtual address of `arch_do_domctl`.

In summary this example patched the callee of the affected function by
 * allocating memory for the new code to live in,
 * changing the virtual address of all the functions which called the old
   code (computing the new offset, patching the callq with a new callq).
 * changing the function pointer tables with the new virtual address of
   the function (splicing in the new virtual address). Since this table
   resides in the .rodata section we would need to temporarily change the
   page table permissions during this part.

However it has severe drawbacks - the safety checks which have to make sure
the function is not on the stack - must also check every caller. For some
patches this could if there were an sufficient large amount of callers
that we would never be able to apply the update.

### Example of different trampoline patching.

An alternative mechanism exists where we can insert an trampoline in the
existing function to be patched to jump directly to the new code. This
lessens the locations to be patched to one but it puts pressure on the
CPU branching logic (I-cache, but it is just one unconditional jump).

For this example we will assume that the hypervisor has not been compiled
with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures
for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure
in `xen_version` hypercall. This function is not called **anywhere** in
the hypervisor (it is called by the guest) but referenced in the
`compat_hypercall_table` and `hypercall_table` (and indirectly called
from that). Patching the offset in `hypercall_table` for the old
`do_xen_version` (ffff82d080112f9e <do_xen_version>)

 ffff82d08024b270 <hypercall_table>  
 ffff82d08024b2f8:   9e 2f 11 80 d0 82 ff ff  

with the new address where the new `do_xen_version` is possible. The other
place where it is used is in `hvm_hypercall64_table` which would need
to be patched in a similar way. This would require an in-place splicing
of the new virtual address of `do_xen_version`.

An alternative solution would be to patch insert an trampoline in the
old `do_xen_version' function to directly jump to the new `do_xen_version`.

 ffff82d080112f9e <do_xen_version>:  
 ffff82d080112f9e:       48 c7 c0 da ff ff ff    mov    
 ffff82d080112fa5:       83 ff 09                cmp    $0x9,%edi  
 ffff82d080112fa8:       0f 87 24 05 00 00       ja     ffff82d0801134d2 


 ffff82d080112f9e <do_xen_version>:  
 ffff82d080112f9e:       e9 XX YY ZZ QQ          jmpq   [new do_xen_version]  

which would lessen the amount of patching to just one location.

In summary this example patched the affected function to jump to the
new replacement function which required:
 * allocating memory for the new code to live in,
 * inserting trampoline with new offset in the old function to point to the
   new function.
 * Optionally we can insert in the old function an trampoline jump to an 
   providing an BUG_ON to catch errant code.

The disadvantage of this are that the unconditional jump will consume a small
I-cache penalty. However the simplicity of the patching of safety checks
make this a worthwhile option.

### Security

With this method we can re-write the hypervisor - and as such we **MUST** be
diligent in only allowing certain guests to perform this operation.

Furthermore with SecureBoot or tboot, we **MUST** also verify the signature
of the payload to be certain it came from a trusted source.

As such the hypercall **MUST** support an XSM policy to limit the what
guest is allowed. If the system is booted with signature checking the
signature checking will be enforced.

## Payload format

The payload **MUST** contain enough data to allow us to apply the update
and also safely reverse it. As such we **MUST** know:

 * What the old code is expected to be. We **MUST** verify it against the
   runtime code.
 * The locations in memory to be patched. This can be determined dynamically
   via symbols or via virtual addresses.
 * The new code to be used.
 * Signature to verify the payload.

This binary format can be constructed using an custom binary format but
there are severe disadvantages of it:

 * The format might need to be change and we need an mechanism to accommodate
 * It has to be platform agnostic.
 * Easily constructed using existing tools.

As such having the payload in an ELF file is the sensible way. We would be
carrying the various set of structures (and data) in the ELF sections under
different names and with definitions. The prefix for the ELF section name
would always be: *.xsplice_*

Note that every structure has padding. This is added so that the hypervisor
can re-use those fields as it sees fit.

There are five sections *.xsplice_* sections:

 * `.xsplice_symbols` and `.xsplice_str`. The array of symbols to be referenced
   during the update. This can contain the symbols (functions) that will be
   patched, or the list of symbols (functions) to be checked pre-patching which
   may not be on the stack.

* `.xsplice_reloc` and `.xsplice_reloc_howto`. The howto properly construct
   trampolines for an patch. We can have multiple locations for which we
   need to insert an trampoline for a payload and each location might require
   a different way of handling it. This would naturally reference the `.text`
   section and its proper offset. The `.xsplice_reloc` is not directly concerned
   with patches but rather is an ELF relocation - describing the target
   of a relocation and how that is performed.  They're also used for where
   the new code references the run code too.

 * `.xsplice_sections`. The safety data for the old code and new code.
   This contains an array of symbols (pointing to `.xsplice_symbols` to
   and `.text`) which are to be used during safety and dependency checking.

 * `.xsplice_patches`: The description of the new functions to be patched
   in (size, type, pointer to code, etc.).

 * `.xsplice_change`. The structure that ties all of this together and defines
   the payload.

Additionally the ELF file would contain:

 * `.text` section for the new and old code (function).
 * `.rela.text` relocation data for the `.text` (both new and old).
 * `.rela.xsplice_patches` relocation data for `.xsplice_patches` (such as 
   to the `.text` ,`.xsplice_symbols`, or `.xsplice_reloc` section).
 * `.bss` section for the new code (function)
 * `.data` and `.data.read_mostly` section for the new and old code (function)
 * `.rodata` section for the new and old code (function).

In short the *.xsplice_* sections represent various structures and the
ELF provides the mechanism to glue it all together when loaded in memory.

Note that a lot of these ideas are borrowed from kSplice which is
available at: https://github.com/jirislaby/ksplice

For ELF understanding the best starting point is the OSDev Wiki
(http://wiki.osdev.org/ELF). Furthermore the ELF specification is
at http://www.skyfree.org/linux/references/ELF_Format.pdf and
at Oracle's web site:

### ASCII art of the ELF structures

*TODO*: Include an ASCII art of how the sections are tied together.

### xsplice_symbols

The section contains an array of an structure that outlines the name
of the symbol to be patched (or checked against). The structure is
as follow:

struct xsplice_symbol {  
    const char *name; /* The ELF name of the symbol. */  
    const char *label; /* A unique xSplice name for the symbol. */  
    uint8_t pad[16]; /* Must be zero. */  
The structures may be in the section in any order and in any amount
(duplicate entries are permitted).

Both `name` and `label` would be pointing to entries in `.xsplice_str`.

The `label` is used for diagnostic purposes - such as including the
name and the offset.

### xsplice_reloc and xsplice_reloc_howto

The section contains an array of a structure that outlines the different
locations (and howto) for which an trampoline is to be inserted.

The howto defines in the detail the change. It contains the type,
whether the relocation is relative, the size of the relocation,
bitmask for which parts of the instruction or data are to be replaced,
amount of final relocation is shifted by (to drop unwanted data), and
whether the replacement should be interpreted as signed value.

The structure is as follow:

#define XSPLICE_HOWTO_RELOC_INLINE  0 /* Inline replacement. */  
#define XSPLICE_HOWTO_RELOC_PATCH   1 /* Add trampoline. */  
#define XSPLICE_HOWTO_RELOC_DATA    2 /*  __DATE__ type change. */  
#define XSPLICE_HOWTO_RELOC_TIME    3 /* __TIME__ type chnage. */  
#define XSPLICE_HOWTO_BUG           4 /* BUG_ON being replaced.*/  
#define XSPLICE_HOWTO_EXTABLE       5 /* exception_table change. */  
#define XSPLICE_HOWTO_SYMBOL        6 /* change in symbol table. */  

#define XSPLICE_HOWTO_FLAG_PC_REL    0x00000001 /* Is PC relative. */  
#define XSPLICE_HOWOT_FLAG_SIGN      0x00000002 /* Should the new value be 
treated as signed value. */  

struct xsplice_reloc_howto {  
    uint32_t    type; /* XSPLICE_HOWTO_* */  
    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
    uint32_t    r_shift; /* The value the final relocation is shifted right by; 
used to drop unwanted data from the relocation. */  
    uint64_t    mask; /* Bitmask for which parts of the instruction or data are 
replaced with the relocated value. */  
    uint8_t     pad[8]; /* Must be zero. */  


This structure is used in:

struct xsplice_reloc {  
    uint64_t addr; /* The address of the relocation (if known). */  
    struct xsplice_symbol *symbol; /* Symbol for this relocation. */  
    struct xsplice_reloc_howto  *howto; /* Pointer to the above structure. */  
    uint64_t isns_added; /* ELF addend resulting from quirks of instruction one 
of whose operands is the relocation. For example, this is -4 on x86 pc-relative 
jumps. */  
    uint64_t isns_target; /* rest of the ELF addend.  This is equal to the 
offset against the symbol that the relocation refers to. */  
    uint8_t pad[8];  /* Must be zero. */  

### xsplice_sections

The structure defined in this section is used to verify that it is safe
to update with the new changes. It can contain safety data on the old code
and what kind of matching we are to expect.

It also can contain safety date of what to check when about to patch.
That is whether any of the addresses (either provided or resolved
when payload is loaded by referencing the symbols) are in memory
with what we expect it to be.

As such the flags can be or-ed together:

#define XSPLICE_SECTION_TEXT   0x00000001 /* Section is in .text */  
#define XSPLICE_SECTION_RODATA 0x00000002 /* Section is in .ro */  
#define XSPLICE_SECTION_DATA   0x00000004 /* Section is in .rodata */  
#define XSPLICE_SECTION_STRING 0x00000008 /* Section is in .str */  
#define XSPLICE_SECTION_ALTINSTRUCTIONS 0x00000010 /* Section has 
.altinstructions. */  
#define XSPLICE_SECTION_TEXT_INPLACE 0x00000200 /* Change is in place. */   
#dekine XSPLICE_SECTION_MATCH_EXACT 0x00000400 /* Must match exactly. */  
#define XSPLICE_SECTION_NO_STACKCHECK 0x00000800 /* Do not check the stack. */  

struct xsplice_section {  
    struct xsplice_symbol *symbol; /* The symbol associated with this change. 
    uint64_t address; /* The address of the section (if known). */  
    uint64_t size; /* The size of the section. */  
    uint64_t flags; /* Various XSPLICE_SECTION_* flags. */
    uint8_t pad[16]; /* To be zero. */  


### xsplice_patches

Within this section we have an array of a structure defining the new code 

This structure consist of an pointer to the new code (which in ELF ends up
pointing to an offset in `.text` or `.data` section); the type of patch:
inline - either text or data, or requesting an trampoline; and size of patch.

The structure is as follow:


struct xsplice_patch {  
    uint32_t type; /* XSPLICE_PATCH_* .*/  
    uint32_t size; /* Size of patch. */  
    uint64_t addr; /* The address of the new code (or data). */  
    void *content; /* The bytes to be installed. */  
    uint8_t pad[16]; /* Must be zero. */  


### xsplice_code

The structure embedded within this section ties it all together.
It has the name of the patch, and pointers to all the above
mentioned structures (the start and end addresses).

The structure is as follow:

struct xsplice_code {  
    const char *name; /* A sensible name for the patch. Up to 40 characters. */ 
    struct xsplice_reloc *relocs, *relocs_end; /* How to patch it */  
    struct xsplice_section *sections, *sections_end; /* Safety data */  
    struct xsplice_patch *patches, *patches_end; /* Patch code & data */  
    uint8_t pad[32]; /* Must be zero. */

There should only be one such structure in the section.

### Example

*TODO*: Include an objdump of how the ELF would look like for the XSA
mentioned earlier.

## Signature checking requirements.

The signature checking requires that the layout of the data in memory
**MUST** be same for signature to be verified. This means that the payload
data layout in ELF format **MUST** match what the hypervisor would be
expecting such that it can properly do signature verification.

The signature is based on the all of the payloads continuously laid out
in memory. The signature is to be appended at the end of the ELF payload
prefixed with the string '~Module signature appended~\n", followed by
an signature header then followed by the signature, key identifier, and signers

Specifically the signature header would be:

#define PKEY_ALGO_DSA       0  
#define PKEY_ALGO_RSA       1  

#define PKEY_ID_PGP         0 /* OpenPGP generated key ID */  
#define PKEY_ID_X509        1 /* X.509 arbitrary subjectKeyIdentifier */  

#define HASH_ALGO_MD4          0  
#define HASH_ALGO_MD5          1  
#define HASH_ALGO_SHA1         2  
#define HASH_ALGO_RIPE_MD_160  3  
#define HASH_ALGO_SHA256       4  
#define HASH_ALGO_SHA384       5  
#define HASH_ALGO_SHA512       6  
#define HASH_ALGO_SHA224       7  
#define HASH_ALGO_RIPE_MD_128  8  
#define HASH_ALGO_RIPE_MD_256  9  
#define HASH_ALGO_RIPE_MD_320 10  
#define HASH_ALGO_WP_256      11  
#define HASH_ALGO_WP_384      12  
#define HASH_ALGO_WP_512      13  
#define HASH_ALGO_TGR_128     14  
#define HASH_ALGO_TGR_160     15  
#define HASH_ALGO_TGR_192     16  

struct elf_payload_signature {  
        u8      algo;           /* Public-key crypto algorithm PKEY_ALGO_*. */  
        u8      hash;           /* Digest algorithm: HASH_ALGO_*. */  
        u8      id_type;        /* Key identifier type PKEY_ID*. */  
        u8      signer_len;     /* Length of signer's name */  
        u8      key_id_len;     /* Length of key identifier */  
        u8      __pad[3];  
        __be32  sig_len;        /* Length of signature data */  

(Note that this has been borrowed from Linux module signature code.).

## Hypercalls

We will employ the sub operations of the system management hypercall (sysctl).
There are to be four sub-operations:

 * upload the payloads.
 * listing of payloads summary uploaded and their state.
 * getting an particular payload summary and its state.
 * command to apply, delete, or revert the payload.

The patching is asynchronous therefore the caller is responsible
to verify that it has been applied properly by retrieving the summary of it
and verifying that there are no error codes associated with the payload.

We **MUST** make it asynchronous due to the nature of patching: it requires
every physical CPU to be lock-step with each other. The patching mechanism
while an implementation detail, is not an short operation and as such
the design **MUST** assume it will be an long-running operation.

Furthermore it is possible to have multiple different payloads for the same
function. As such an unique id has to be visible to allow proper manipulation.

The hypercall is part of the `xen_sysctl`. The top level structure contains
one uint32_t to determine the sub-operations:

struct xen_sysctl_xsplice_op {  
    uint32_t cmd;  
        union {  
          ... see below ...  
        } u;  

while the rest of hypercall specific structures are part of the this structure.


Upload a payload to the hypervisor. The payload is verified and if there
are any issues the proper return code will be returned. The payload is
not applied at this time - that is controlled by *XEN_SYSCTL_XSPLICE_ACTION*.

The caller provides:

 * `id` unique id.
 * `payload` the virtual address of where the ELF payload is.

The return value is zero if the payload was succesfully uploaded and the
signature was verified. Otherwise an EXX return value is provided.
Duplicate `id` are not supported.

The `payload` is the ELF payload as mentioned in the `Payload format` section.

The structure is as follow:

struct xen_sysctl_xsplice_upload {  
    char id[40];  /* IN, name of the patch. */  
    uint64_t size; /* IN, size of the ELF file. */
    XEN_GUEST_HANDLE_64(uint8) payload; /* ELF file. */  


Retrieve an summary of an specific payload. This caller provides:

 * `id` the unique id.
 * `status` *MUST* be set to zero.
 * `rc` *MUST* be set to zero.

The `summary` structure contains an summary of payload which includes:

 * `id` the unique id.
 * `status` - whether it has been:
 1. *XSPLICE_STATUS_LOADED* (0) has been loaded.
 3. *XSPLICE_STATUS_CHECKED*  (2) the ELF payload safety checks passed.
 4. *XSPLICE_STATUS_APPLIED* (3) loaded, checked, and applied.
 5. *XSPLICE_STATUS_REVERTED* (4) loaded, checked, applied and then also 
 6. *XSPLICE_STATUS_IN_ERROR* (5) loaded and in a failed state. Consult `rc` 
for details.
 * `rc` - its error state if any.

The structure is as follow:


struct xen_sysctl_xsplice_summary {  
    char id[40];  /* IN/OUT, name of the patch. */  
    uint32_t status;   /* OUT */  
    int32_t rc;  /* OUT */  


Retrieve an array of abbreviated summary of payloads that are loaded in the

The caller provides:

 * `idx` index iterator. Initially it *MUST* be zero.
 * `count` the max number of entries to populate.
 * `summary` virtual address of where to write payload summaries.

The hypercall returns zero on success and updates the `idx` (index) iterator
with the number of payloads returned, `count` to the number of remaining
payloads, and `summary` with an number of payload summaries.

If the hypercall returns E2BIG the `count` is too big and should be

Note that due to the asynchronous nature of hypercalls the domain might have
added or removed the number of payloads making this information stale. It is
the responsibility of the domain to provide proper accounting.

The `summary` structure contains an summary of payload which includes:

 * `id` unique id.
 * `status` - whether it has been:
 1. *XSPLICE_STATUS_LOADED* (0) has been loaded.
 3. *XSPLICE_STATUS_CHECKED*  (2) the payload `old` and `addr` match with the 
 4. *XSPLICE_STATUS_APPLIED* (3) loaded, checked, and applied.
 5. *XSPLICE_STATUS_REVERTED* (4) loaded, checked, applied and then also 
 6. *XSPLICE_STATUS_IN_ERROR* (5) loaded and in a failed state. Consult `rc` 
for details.
 * `rc` - its error state if any.

The structure is as follow:

struct xen_sysctl_xsplice_list {  
    uint32_t idx;  /* IN/OUT */  
    uint32_t count;  /* IN/OUT */
    XEN_GUEST_HANDLE_64(xen_sysctl_xsplice_summary) summary;  /* OUT */  

struct xen_sysctl_xsplice_summary {  
    char id[40];  /* OUT, name of the patch. */  
    uint32_t status;   /* OUT */  
    int32_t rc;  /* OUT */  


Perform an operation on the payload structure referenced by the `id` field.
The operation request is asynchronous and the status should be retrieved

The caller provides:

 * `id` the unique id.
 * `cmd` the command requested:
  1. *XSPLICE_ACTION_CHECK* (0) check that the payload will apply properly.
  2. *XSPLICE_ACTION_UNLOAD* (1) unload the payload.  
  3. *XSPLICE_ACTION_REVERT* (2) revert the payload.  
  4. *XSPLICE_ACTION_APPLY* (3) apply the payload.   

The return value will be zero unless the provided fields are incorrect.

The structure is as follow:


struct xen_sysctl_xsplice_action {  
    char id[40];  /* IN, name of the patch. */  
    uint32_t cmd; /* IN */  


## Sequence of events.

The normal sequence of events is to:

 1. *XEN_SYSCTL_XSPLICE_UPLOAD* to upload the payload. If there are errors 
*STOP* here.
 2. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in 
that the payload can be succesfully applied.
 4. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in 
 6. *XEN_SYSCTL_XSPLICE_GET* to check the `->status`. If in 

## Addendum

Implementation quirks should not be discussed in a design document.

However these observations can provide aid when developing against this

### Alternative assembler

Alternative assembler is a mechanism to use different instructions depending
on what the CPU supports. This is done by providing multiple streams of code
that can be patched in - or if the CPU does not support it - padded with
`nop` operations. The alternative assembler macros cause the compiler to
expand the code to place a most generic code in place - emit a special
ELF .section header to tag this location. During run-time the hypervisor
can leave the areas alone or patch them with an better suited opcodes.

As we might be patching the alternative assembler sections as well - by
providing a new better suited op-codes or perhaps with nops - we need to
also re-run the alternative assembler patching after we have done our

Also when we are doing safety checks the code we are checking might be
utilizing alternative assembler. As such we should relax out checks to
accomodate that.

### .rodata sections

The patching might require strings to be updated as well. As such we must be
also able to patch the strings as needed. This sounds simple - but the compiler
has a habit of coalescing strings that are the same - which means if we in-place
alter the strings - other users will be inadvertently affected as well.

This is also where pointers to functions live - and we may need to patch this
as well.

To guard against that we must be prepared to do patching similar to
trampoline patching or in-line depending on the flavour. If we can
do in-line patching we would need to:

 * alter `.rodata` to be writeable.
 * inline patch.
 * alter `.rodata` to be read-only.

If are doing trampoline patching we would need to:

 * allocate a new memory location for the string.
 * all locations which use this string will have to be updated to use the
   offset to the string.
 * mark the region RO when we are done.

### .bss sections

Patching writable data is not suitable as it is unclear what should be done
depending on the current state of data. As such it should not be attempted.

### Patching code which is in the stack.

We should not patch the code which is on the stack. That can lead
to corruption.

### Trampoline (e9 opcode)

The e9 opcode used for jmpq uses a 32-bit signed displacement. That means
we are limited to up to 2GB of virtual address to place the new code
from the old code. That should not be a problem since Xen hypervisor has
a very small footprint.

However if we need - we can always add two trampolines. One at the 2GB
limit that calls the next trampoline.

### Time rendezvous code instead of stop_machine for patching

The hypervisor's time rendezvous code runs synchronously across all CPUs
every second. Using the stop_machine to patch can stall the time rendezvous
code and result in NMI. As such having the patching be done at the tail
of rendezvous code should avoid this problem.

### Security

Only the privileged domain should be allowed to do this operation.

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