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Re: [Qemu-devel] [PATCH 4/6] linux-user: Provide safe_syscall for aarch6

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH 4/6] linux-user: Provide safe_syscall for aarch64
Date: Mon, 13 Jun 2016 15:21:14 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0

On 06/13/2016 03:04 PM, Peter Maydell wrote:
+        .global safe_syscall_base
+        .global safe_syscall_start
+        .global safe_syscall_end
+        .type   safe_syscall_base, #function
+        .type   safe_syscall_start, #function
+        .type   safe_syscall_end, #function

_start and _end aren't function entry points, so is it OK
to mark them as functions?

Yes. Indeed, if you don't, the objdump will think these are data symbols and fail to disassemble the instructions that follow.

(The 'as' manual doesn't document what setting .type does in much

It sets the Elf_Sym.st_type field.
But what that implies is very host specific.

doesn't mention a syscall argument in x7, just x0..x5.

I took glibc as definitive, since that's code that definitely works.


I'll also point you at the kernel's __sys_trace, which does in fact save and restore all 8 registers around the tracing call-outs.

+       mov     x9, x0          /* signal_pending pointer */
+       mov     w8, w1          /* syscall number */

Seems a bit odd to use a 32-bit move for this when our input
calling convention has it as 64 bits and the kernel's calling
convention has it as 64 bits.

I got that from glibc too.


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