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Re: [PATCH v6 1/3] xen/arm: add support for run_in_exception_handler()

Hi Juergen,

On 16/01/2021 10:33, Juergen Gross wrote:
Add support to run a function in an exception handler for Arm. Do it
as on x86 via a bug_frame, but pass the function pointer via a
register (this needs to be done that way, because inline asm support
for 32-bit Arm lacks the needed functionality to reference an
arbitrary function via the bugframe).

Use the same BUGFRAME_* #defines as on x86 in order to make a future
common header file more easily achievable.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
- new patch

- adjust BUG_FRAME() macro (Jan Beulich)
- adjust arm linker script (Jan Beulich)
- drop #ifdef x86 in common/virtual_region.c

- use register for function address (Arm32 build failure noticed by
   Julien Grall)

Thank you for trying to sort out the issue on Arm32 :).

+ * Unfortunately gcc for arm 32-bit doesn't support the "i" constraint, so
+ * the easiest way to implement run_in_exception_handler() is to pass the
+ * to be called function in a fixed register.

There are a few uses of "i" in Linux Arm32 (such as jump label), therefore I am pretty confident "i" works at least in some situation.

I did some more digging this afternoon. Our use of "i" is very similar to Linux, so I decided to look at the GCC flags used.

It turns out that Linux will always build with -fno-pie on Arm (either 32 or 64) whereas Xen will let the compiler to decide (PIE is enabled by on my compiler).

I wrote a small example to test the issue (based on Linux static key)

static void test(void)

int main(void)
    asm volatile (".pushsection .bug_frames, \"aw\"\n"
              ".quad %0\n"
              :: "i"(test));

    return 0;

If I add -fno-pie on the command, the constraint error disappears.

On the previous version, you rewrote that didn't see any error. Is it possible that your compiler is disabling PIE by default?

I think we need to code to be position independent for at least UEFI. I also have plan to look at selecting the Xen virtual address at boot time (this is necessary to solve some memory issue on Arm).

From a quick test, if I use -fno-pie -fpic, then the snipped above will build fine. But it is not entirely clear whether the code would still be position independent.

I need to have a look how Linux is dealing with KASLR given that -fno-pie is used...

+ */
+#define  run_in_exception_handler(fn) do {                                  \
+    asm ("mov " __stringify(BUG_FN_REG) ", %0\n"                            \
+         "1:"BUG_INSTR"\n"                                                  \
+         ".pushsection .bug_frames." __stringify(BUGFRAME_run_fn) ","       \
+         "             \"a\", %%progbits\n"                                 \
+         "2:\n"                                                             \
+         ".p2align 2\n"                                                     \
+         ".long (1b - 2b)\n"                                                \
+         ".long 0, 0, 0\n"                                                  \
+         ".popsection" :: "r" (fn) : __stringify(BUG_FN_REG) );             \
+} while (0)

My concern with this approach is we are going to clobber multiple registers. On Arm64, this code will be replaced by:

    13cc:       90000001        adrp    x1, 0 <show_execution_state>
    13d0:       91000021        add     x1, x1, #0x0
    13d4:       aa0103e0        mov     x0, x1
    13d8:       d4200020        brk     #0x1

I guess we can optimize it to just clobber one register. Do we expect the function executed to rely/replace the content of the registers?


Julien Grall



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