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

Bhupesh Sharma
2025-08-05T11:14:18Z

On 7/27/25 5:07 AM, Linus Torvalds wrote:
> On Sat, 26 Jul 2025 at 16:19, Kees Cook <kees@kernel.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