Re: [Xen-devel] [PATCH v6 3/6] livepatch: NOP if func->new_addr is zero.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Sep 19, 2016 at 10:31:23AM -0600, Jan Beulich wrote:
> >>> On 19.09.16 at 18:11, <konrad.wilk@xxxxxxxxxx> wrote:
> > On Mon, Sep 19, 2016 at 02:59:32AM -0600, Jan Beulich wrote:
> >> >>> On 16.09.16 at 17:29, <konrad.wilk@xxxxxxxxxx> wrote:
> >> > @@ -31,11 +30,11 @@ void arch_livepatch_revive(void)
> >> >  
> >> >  int arch_livepatch_verify_func(const struct livepatch_func *func)
> >> >  {
> >> > -    /* No NOP patching yet. */
> >> > -    if ( !func->new_size )
> >> > +    /* If NOPing only do up to maximum amount we can put in the 
> >> > ->opaque. */
> >> > +    if ( !func->new_addr && func->new_size > sizeof(func->opaque) )
> >> >          return -EOPNOTSUPP;
> >> >  
> >> > -    if ( func->old_size < PATCH_INSN_SIZE )
> >> > +    if ( func->old_size < ARCH_PATCH_INSN_SIZE )
> >> >          return -EINVAL;
> >> 
> >> Is that indeed a requirement when NOPing? You can easily NOP out
> >> just a single byte on x86. Or shouldn't in that case old_size == new_size
> >> anyway? In which case the comment further down stating that new_size
> > 
> > The original intent behind .old_size was to guard against patching
> > functions that were less than our relative jump. 
> > 
> > (The tools end up computing the .old_size as the size of the whole function
> > which is fine).
> > 
> > But with this NOPing support, you are right - we could have now an
> > function that is say 4 bytes long and we only need to NOP three bytes
> > out of it (the last instruction I assume would be 'ret').
> > 
> > So perhaps this check needs just needs an 'else if' , like so:
> > 
> > int arch_livepatch_verify_func(const struct livepatch_func *func)
> > {
> >     /* If NOPing.. */
> >     if ( !func->new_addr )
> >     {
> >         /* Only do up to maximum amount we can put in the ->opaque. */
> >         if ( func->new_size > sizeof(func->opaque) )
> >             return -EOPNOTSUPP;
> > 
> >         /* One instruction for 'ret' and the other to NOP. */
> >         if ( func->old_size < 2 )
> >             return -EINVAL;
> >     }
> >     else if ( func->old_size < ARCH_PATCH_INSN_SIZE )
> >         return -EINVAL;
> > 
> >     return 0;
> > }
> 
> Except that I wouldn't use 2, to not exclude patching out some
> single byte in the middle of a function, without regard to what the
> function's actual size is.

Uh-uh.

The _new_size_ determines how many bytes to NOP (in the context of
this patch). The old_size (where we check to be at min 2) is a safety
valve to make sure we don't NOP something outside the function.

..snip..
> >> NOP addition here, perhaps worth dropping the _jmp from the
> >> function name (and its revert counterpart)?
> > 
> > Ooh, good idea. But I think it maybe better as a seperate patch (as it
> > also touches the ARM code).
> 
> That's in the other series, isn't it?

It expands the existing ones. Right now in 'staging' branch we have an
arch/arm/livepatch.c which has these functions in it.

Granted nothing compiles them, so I could break it in this patch.

But I already cobbled up the patch so may as well use it?

From 45abdd6a0c3a6a2ca7180c7340032ac5b2b186a4 Mon Sep 17 00:00:00 2001
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Mon, 19 Sep 2016 12:20:27 -0400
Subject: [PATCH] livepatch: Drop _jmp from arch_livepatch_[apply,revert]_jmp

With "livepatch: NOP if func->new_addr is zero." that name
makes no more sense.

Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
v7: New submission
---
 xen/arch/arm/livepatch.c    | 4 ++--
 xen/arch/x86/livepatch.c    | 4 ++--
 xen/include/xen/livepatch.h | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
index 755f596..7f067a0 100644
--- a/xen/arch/arm/livepatch.c
+++ b/xen/arch/arm/livepatch.c
@@ -21,11 +21,11 @@ int arch_livepatch_verify_func(const struct livepatch_func 
*func)
     return -ENOSYS;
 }
 
-void arch_livepatch_apply_jmp(struct livepatch_func *func)
+void arch_livepatch_apply(struct livepatch_func *func)
 {
 }
 
-void arch_livepatch_revert_jmp(const struct livepatch_func *func)
+void arch_livepatch_revert(const struct livepatch_func *func)
 {
 }
 
diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
index 118770e..36bbc5f 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -47,7 +47,7 @@ int arch_livepatch_verify_func(const struct livepatch_func 
*func)
     return 0;
 }
 
-void arch_livepatch_apply_jmp(struct livepatch_func *func)
+void arch_livepatch_apply(struct livepatch_func *func)
 {
     uint8_t *old_ptr;
     uint8_t insn[sizeof(func->opaque)];
@@ -76,7 +76,7 @@ void arch_livepatch_apply_jmp(struct livepatch_func *func)
     memcpy(old_ptr, insn, len);
 }
 
-void arch_livepatch_revert_jmp(const struct livepatch_func *func)
+void arch_livepatch_revert(const struct livepatch_func *func)
 {
     memcpy(func->old_addr, func->opaque, livepatch_insn_len(func));
 }
diff --git a/xen/include/xen/livepatch.h b/xen/include/xen/livepatch.h
index 174af06..b7f66d4 100644
--- a/xen/include/xen/livepatch.h
+++ b/xen/include/xen/livepatch.h
@@ -86,8 +86,8 @@ unsigned int livepatch_insn_len(const struct livepatch_func 
*func)
 int arch_livepatch_quiesce(void);
 void arch_livepatch_revive(void);
 
-void arch_livepatch_apply_jmp(struct livepatch_func *func);
-void arch_livepatch_revert_jmp(const struct livepatch_func *func);
+void arch_livepatch_apply(struct livepatch_func *func);
+void arch_livepatch_revert(const struct livepatch_func *func);
 void arch_livepatch_post_action(void);
 
 void arch_livepatch_mask(void);
-- 
2.4.11

> 
> Jan
> 

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