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

[Bhupesh Sharma]

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