Re: [PATCH next 1/1] fs: Mark get_sigset_argpack() __always_inline

[Linus Torvalds]

On Sat, 8 Feb 2025 at 11:06, David Laight <david.laight.linux@xxxxxxxxx> wrote:
>
> Can the 'alternatives' be flipped so the .o doesn't contain loads of nops?

Sadly, no. The instructions generate #UD if the CPU doesn't support SMAP.

Now, arguably the alternatives *should* be fixed up before the first
user space access and thus it would be safe to switch, but honestly, I
don't want to risk some early code doing odd things. The potential
pain would be too damn high.

> It'd be nice to see the clac and lfence.

Heh. I agree 100%, which is why my personal tree has a patch like this:

    -#define LOCK_PREFIX_HERE \
    -           ".pushsection .smp_locks,\"a\"\n"       \
    -           ".balign 4\n"                           \
    -           ".long 671f - .\n" /* offset */         \
    -           ".popsection\n"                         \
    -           "671:"
    +#define LOCK_PREFIX_HERE
  ...
    -#define barrier_nospec() alternative("", "lfence",
X86_FEATURE_LFENCE_RDTSC)
    +#define barrier_nospec() asm volatile("lfence":::"memory")
  ...
    -#define ASM_CLAC \
    -   ALTERNATIVE "", "clac", X86_FEATURE_SMAP
    -
    -#define ASM_STAC \
    -   ALTERNATIVE "", "stac", X86_FEATURE_SMAP
    +#define ASM_CLAC   clac
    +#define ASM_STAC   stac
  ...
    -   /* Note: a barrier is implicit in alternative() */
    -   alternative("", "clac", X86_FEATURE_SMAP);
    +   asm volatile("clac":::"memory");
  ...
    -   /* Note: a barrier is implicit in alternative() */
    -   alternative("", "stac", X86_FEATURE_SMAP);
    +   asm volatile("stac":::"memory");
  ...
    -#define ASM_BARRIER_NOSPEC ALTERNATIVE "", "lfence",
X86_FEATURE_LFENCE_RDTSC
    +#define ASM_BARRIER_NOSPEC lfence

but I've never made it a real usable patch for others. It would have
to be some "modern CPU only" config variable, and nobody else has ever
complained about this until now.

             Linus