BusyBox 1.36.0 regression: Segfaults on i386 musl libc

[Sören Tempel]

Investigated this further. The problem is a text relocation created by
the hash_md5_sha256_x86-32_shaNI.S file. When compiling BusyBox with
LDFLAGS=-Wl,-z,text one is warned about the following relocation by gcc:

	/usr/lib/gcc/i586-alpine-linux-musl/12.2.1/../../../../i586-alpine-linux-musl/bin/ld: libbb/lib.a(hash_md5_sha_x86-32_shaNI.o): warning: relocation in read-only section `.text.sha1_process_block64_shaNI'
	/usr/lib/gcc/i586-alpine-linux-musl/12.2.1/../../../../i586-alpine-linux-musl/bin/ld: read-only segment has dynamic relocations

The Linux Kernel, from which the assembly was copied, does addressing
relative to the %pic register to avoid this relocation it seems [1]:

	movdqa PSHUFFLE_BYTE_FLIP_MASK(%rip), SHUF_MASK

However, the %rip register is AFAIK not available for i386 and since I
am personally not an x86 wizard I have no idea how to best rewrite this
code in a way that it doesn't require dynamic relocations.

[1]: https://github.com/torvalds/linux/blob/94a855111ed9106971ca2617c5d075269e6aefde/arch/x86/crypto/sha1_ni_asm.S#L112

Sören Tempel <soeren at soeren-tempel.net> wrote:
> Hello,
> 
> Natanael Copa <ncopa at alpinelinux.org> wrote:
> > diff --git a/libbb/hash_md5_sha.c b/libbb/hash_md5_sha.c
> > index 880ffab01..d2351d3e6 100644
> > --- a/libbb/hash_md5_sha.c
> > +++ b/libbb/hash_md5_sha.c
> > @@ -17,8 +17,11 @@
> >  # if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
> >  static void cpuid(unsigned *eax, unsigned *ebx, unsigned *ecx, unsigned *edx)
> >  {
> > -       asm ("cpuid"
> > -               : "=a"(*eax), "=b"(*ebx), "=c"(*ecx), "=d"(*edx)
> > +       asm volatile (
> > +               "mov %%ebx, %%esi;" /* save %ebx PIC register */
> > +               "cpuid;"
> > +               "xchg %%ebx, %%esi;"
> > +               : "=a"(*eax), "=S"(*ebx), "=c"(*ecx), "=d"(*edx)
> >                 : "0"(*eax),  "1"(*ebx),  "2"(*ecx),  "3"(*edx)
> >         );
> >  }
> 
> Unfortunately, this does not fix the segfault. Since the segfault occurs
> in musl's dynamic loader I also don't think that this code is
> reached/executed. Instead, this seems to be a problem with the symbols
> of the provided assembly file.
> 
> I am currently debugging this on a96ccbefe417aaac6a2ce59c788e01fc0f83902f.
> If I remove the PSHUFFLE_BYTE_FLIP_MASK definition (and the instruction
> using it) in hash_md5_sha256_x86-32_shaNI.S from the checkout for this
> commit then the segfault doesn't occur. So this does definitely seem to
> be a problem with the hash_md5_sha256_x86-32_shaNI.S assembly file...
> 
> Greetings,
> Sören
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