linux-kernel - Re: [PATCH] powerpc/signal32: Force inlining of __unsafe_save_user_regs() and save_tm_user_regs

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Message-ID: <874jnghjjw.fsf@mail.lhotse>
Date:   Fri, 09 Jun 2023 23:06:11 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/signal32: Force inlining of
 __unsafe_save_user_regs() and save_tm_user_regs_unsafe()

"Nicholas Piggin" <npiggin@...il.com> writes:
> On Mon Jun 5, 2023 at 6:58 PM AEST, Christophe Leroy wrote:
>> Looking at generated code for handle_signal32() shows calls to a
>> function called __unsafe_save_user_regs.constprop.0 while user access
>> is open.
>>
>> And that __unsafe_save_user_regs.constprop.0 function has two nops at
>> the begining, allowing it to be traced, which is unexpected during
>> user access open window.
>>
>> The solution could be to mark __unsafe_save_user_regs() no trace, but
>> to be on the safe side the most efficient is to flag it __always_inline
>> as already done for function __unsafe_restore_general_regs(). The
>> function is relatively small and only called twice, so the size
>> increase will remain in the noise.
>>
>> Do the same with save_tm_user_regs_unsafe() as it may suffer the
>> same issue.
>
> Could you put a comment so someone doesn't uninline it later?

I think the "unsafe" in the name is probably sufficient to warn people
off, but you never know. Still I'd happily take a patch to add comments :)

> Marking it notrace as well would be sufficient for a comment, if that works.

I nearly did that when applying, but I'm not sure it won't change the
code generation, so I left it as-is.

cheers