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

[Ammar Faizi]

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@xxxxxxxxxxxxx/
> > Link: https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/25
> >
> > This adds info about registers on exit.
> >
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Michael Matz <matz@xxxxxxx>
> > Cc: "H.J. Lu" <hjl.tools@xxxxxxxxx>
> > Cc: Willy Tarreau <w@xxxxxx>
> > Cc: x86-ml <x86@xxxxxxxxxx>
> > Cc: lkml <linux-kernel@xxxxxxxxxxxxxxx>
> > Cc: GNU/Weeb Mailing List <gwml@xxxxxxxxxxx>
> > Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
> > ---
[...]
> > 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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