On Thu, 05 Jan 2023 21:39:09 +0100
Sören Tempel <soe...@soeren-tempel.net> wrote:
> Hi,
>
> I am the maintainer of the BusyBox package for Alpine Linux. While
> upgrading that package from 1.35.0 to 1.36.0 I noticed a segfault
> on Alpine x86, on all other architectures BusyBox 1.36.0 builds
> fine and passes the tests. On x86 though it segfaults with any
> command-line argument, for example:
>
> $ make defconfig
> $ make
> $ gdb --args ./busybox_unstripped
> (gdb) run
> Starting program:
> /home/buildozer/aports/main/busybox/src/build-dynamic/busybox_unstripped
>
> Program received signal SIGSEGV, Segmentation fault.
> 0xf7fc24e0 in do_relocs (dso=dso@entry=0xf7ffca20 <app>,
> rel=0x565578e4, rel_size=8712, stride=2) at ldso/dynlink.c:471
> 471 ldso/dynlink.c: No such file or directory.
> (gdb) bt
> #0 0xf7fc24e0 in do_relocs (dso=dso@entry=0xf7ffca20 <app>,
> rel=0x565578e4, rel_size=8712, stride=2) at ldso/dynlink.c:471
> #1 0xf7fc263f in reloc_all (p=p@entry=0xf7ffca20 <app>) at
> ldso/dynlink.c:1375
> #2 0xf7fc473e in __dls3 (sp=0xffffdcf0, auxv=0xffffdd3c) at
> ldso/dynlink.c:1974
> #3 0xf7fc3eab in __dls2 (base=<optimized out>, sp=<optimized out>) at
> ldso/dynlink.c:1719
> #4 0xf7fc19c9 in _dlstart () from /lib/ld-musl-i386.so.1
>
> Looking at the backtrace, it seems that it segfaults in musl's dynamic
> loader. Since BusyBox 1.35.0 worked fine on x86 I bisected this and it
> turns out that this is a regression introduced in commit
> a96ccbefe417aaac6a2ce59c788e01fc0f83902f [1]. If I disable SHA/MD5
> hardware acceleration then BusyBox 1.36.0 builds fine and passes all
> tests on Alpine Linux x86.
>
> Any idea what particular part of the referenced commit might be causing this?
I believe this happens due to ebx is clobbered which is needed for
position independent code (PIC) on 32 bit x86.
I also wonder if the asm needs to be volatile.
Try something like this:
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)
);
}
>
> Greetings,
> Sören
>
> [1]:
> https://git.busybox.net/busybox/commit/?id=a96ccbefe417aaac6a2ce59c788e01fc0f83902f
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