linux-kernel - Re: [PATCH v6 2/3] treewide: Switch memcpy() users of 'task->comm' to a more safer implementation

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Message-ID: <dfdc2c2c-1486-a84d-9ca2-3b48b68773ce@igalia.com>
Date: Tue, 5 Aug 2025 16:43:58 +0530
From: Bhupesh Sharma <bhsharma@...lia.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Kees Cook <kees@...nel.org>
Cc: Bhupesh <bhupesh@...lia.com>, akpm@...ux-foundation.org,
 kernel-dev@...lia.com, linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 linux-perf-users@...r.kernel.org, linux-fsdevel@...r.kernel.org,
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Subject: Re: [PATCH v6 2/3] treewide: Switch memcpy() users of 'task->comm' to
 a more safer implementation


On 7/27/25 5:07 AM, Linus Torvalds wrote:
> On Sat, 26 Jul 2025 at 16:19, Kees Cook <kees@...nel.org> wrote:
>> That works for me! I just get twitchy around seeing memcpy used for strings. :) if we're gonna NUL after the memcpy, just use strscpy_pad().
> I do worry a tiny bit about performance.
>
> Because 'memcpy+set last byte to NUL' really is just a couple of
> instructions when we're talking small constant-sized arrays.
>
> strscpy_pad() isn't horrible, but it's still at another level. And
> most of the cost is that "return the length" which people often don't
> care about.
>
> Dang, I wish we had some compiler trick to say "if the value isn't
> used, do X, if it _is_ used do Y".
>
> It's such a trivial thing in the compiler itself, and the information
> is there, but I don't think it is exposed in any useful way.
>
> In fact, it *is* exposed in one way I can think of:
>
>     __attribute__((__warn_unused_result__))
>
> but not in a useful form for actually generating different code.
>
> Some kind of "__builtin_if_used(x,y)" where it picks 'x' if the value
> is used, and 'y' if it isn't would be lovely for this.
>
> Then you could do things like
>
>      #define my_helper(x) \
>          __builtin_if_used( \
>                  full_semantics(x), \
>                  simpler_version(x))
>
> when having a return value means extra work and most people don't care.
>
> Maybe it exists in some form that I haven't thought of?
>
> Any compiler people around?
>

Sorry for the delay in reply, but I was checking with some *compiler* 
folks and unfortunately couldn't find an equivalent of the above 
*helper* support.
I am not a compiler expert though and relied mostly on my digging of the 
'gcc' code and advise from folks working in compiler world.

In case there are no new suggestions, I think we can go ahead with 
"strscpy_pad()" or "get_task_array()" in place of "get_task_comm()" 
which is implement in the following manner:

    static __always_inline void
        __cstr_array_copy(char *dst,
             const char *src, __kernel_size_t size)
    {
         memcpy(dst, src, size);
         dst[size] = 0;
    }

    #define get_task_array(a,b) \
       __cstr_array_copy(dst, src, __must_be_array(dst))

Please let me know.

Thanks,
Bhupesh