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

[Andy Lutomirski <luto@kernel.org>]

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>
> ---
> 
> Quoted the full comment in that file after patched, so it's easier to
> review:
> /*
>   * 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
>   *
>   * This is the only entry point used for 64-bit system calls.  The
>   * hardware interface is reasonably well designed and the register to
>   * argument mapping Linux uses fits well with the registers that are
>   * available when SYSCALL is used.
>   *
>   * SYSCALL instructions can be found inlined in libc implementations as
>   * well as some other programs and libraries.  There are also a handful
>   * of SYSCALL instructions in the vDSO used, for example, as a
>   * clock_gettimeofday fallback.
>   *
>   * 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
>   * then loads new ss, cs, and rip from previously programmed MSRs.
>   * rflags gets masked by a value from another MSR (so CLD and CLAC
>   * are not needed). SYSCALL does not save anything on the stack
>   * and does not change rsp.
>   *
>   * Registers on entry:
>   * rax  system call number
>   * rcx  return address
>   * r11  saved rflags (note: r11 is callee-clobbered register in C ABI)
>   * rdi  arg0
>   * rsi  arg1
>   * rdx  arg2
>   * r10  arg3 (needs to be moved to rcx to conform to C ABI)
>   * r8   arg4
>   * r9   arg5
>   * (note: r12-r15, rbp, rbx are callee-preserved in C ABI)
>   *
>   * 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.
>   *
>   * When user can change pt_regs->foo always force IRET. That is because
>   * it deals with uncanonical addresses better. SYSRET has trouble
>   * with them due to bugs in both AMD and Intel CPUs.
>   */
> 
> ---
> 
>   arch/x86/entry/entry_64.S | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> 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.

>    * When user can change pt_regs->foo always force IRET. That is because
>    * it deals with uncanonical addresses better. SYSRET has trouble
>    * with them due to bugs in both AMD and Intel CPUs.
> 

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