linux-kernel - Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation

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Date:   Thu, 5 Oct 2023 10:22:37 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Brian Gerst <brgerst@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Mika Penttilä <mpenttil@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Uros Bizjak <ubizjak@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation


* Brian Gerst <brgerst@...il.com> wrote:

> This patch set cleans up the tests done to determine if a fast syscall
> return instruction can be used to return to userspace.  It converts the
> code to C, and refactors existing code to be more readable.
> 
> v2:
>  - Fix shift value for canonical RIP test and use
>    __is_canonical_address()
> 
> Brian Gerst (6):
>   x86/entry/64: Remove obsolete comment on tracing vs. SYSRET
>   x86/entry/64: Convert SYSRET validation tests to C
>   x86/entry/compat: Combine return value test from syscall handler
>   x86/entry/32: Convert do_fast_syscall_32() to bool return type
>   x86/entry/32: Remove SEP test for SYSEXIT
>   x86/entry/32: Clean up syscall fast exit tests
> 
>  arch/x86/entry/common.c          | 99 +++++++++++++++++++++-----------
>  arch/x86/entry/entry_32.S        |  2 +-
>  arch/x86/entry/entry_64.S        | 68 +---------------------
>  arch/x86/entry/entry_64_compat.S | 12 ++--
>  arch/x86/include/asm/syscall.h   |  6 +-
>  5 files changed, 77 insertions(+), 110 deletions(-)

Ok, so I've applied patches #1, #3, #4 and #5 to tip:x86/entry,
(ie. skipped #2 & #6 for now), as they look correct and are good
improvements. None of these four patches depend on the skipped
patches in some way I missed, correct?

As for #2, I looked at the before/after disassembly, and the new
C code in do_syscall_64() looked suboptimal on x86-64 defconfig,
if I was reading it right.

Mind re-evaluating that, and if you still think the C conversion
is a good idea, mind putting a before/after analysis of the
generated instructions into the changelog? This is our primary
system call return path after all.

Thanks,

	Ingo