Re: [PATCH v1 2/3] x86/entry/64: Add info about registers on exit

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

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On 1/8/22 7:03 AM, Andy Lutomirski wrote:
> On 1/7/22 15:52, Ammar Faizi wrote:
>> There was a controversial discussion about the wording in the System
>> V ABI document regarding what registers the kernel is allowed to
>> clobber when the userspace executes syscall.
>>
>> The resolution of the discussion was reviewing the clobber list in
>> the glibc source. For a historical reason in the glibc source, the
>> kernel must restore all registers before returning to the userspace
>> (except for rax, rcx and r11).
>>
>> Link: https://lore.kernel.org/lkml/alpine.LSU.2.20.2110131601000.26294@wotan.suse.de/
>> Link: https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/25
>>
>> This adds info about registers on exit.
>>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Michael Matz <matz@suse.de>
>> Cc: "H.J. Lu" <hjl.tools@gmail.com>
>> Cc: Willy Tarreau <w@1wt.eu>
>> Cc: x86-ml <x86@kernel.org>
>> Cc: lkml <linux-kernel@vger.kernel.org>
>> Cc: GNU/Weeb Mailing List <gwml@gnuweeb.org>
>> Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
>> ---
[...]
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index e432dd075291..1111fff2e05f 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -79,6 +79,19 @@
>>    *
>>    * Only called from user space.
>>    *
>> + * Registers on exit:
>> + * rax  syscall return value
>> + * rcx  return address
>> + * r11  rflags
>> + *
>> + * For a historical reason in the glibc source, the kernel must restore all
>> + * registers except the rax (syscall return value) before returning to the
>> + * userspace.
>> + *
>> + * In other words, with respect to the userspace, when the kernel returns
>> + * to the userspace, only 3 registers are clobbered, they are rax, rcx,
>> + * and r11.
>> + *
> 
> I would say this much more concisely:
> 
> The Linux kernel preserves all registers (even C callee-clobbered
> registers) except for rax, rcx and r11 across system calls, and
> existing user code relies on this behavior.

Agree, I will take that as Suggested-by in the v2.

-- 
Ammar Faizi


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