musl - Symbol versioning approximation trips on compat symbols

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Message-ID: <87r2d5evvi.fsf@oldenburg2.str.redhat.com>
Date: Mon, 21 Jan 2019 18:57:53 +0100
From: Florian Weimer <fweimer@...hat.com>
To: musl@...ts.openwall.com
Subject: Symbol versioning approximation trips on compat symbols

On what appears to be current Alpine Linux (musl-1.1.19-r10), the
following reproducer

######################################################################
cat > symver.c <<EOF
void
compat_function (void)
{
}
__asm__ (".symver compat_function,compat_function@...VER");

void
call_compat_function (void)
{
  return compat_function ();
}
EOF

echo "SYMVER { };" > symver.map

cat > main.c <<EOF
extern void call_compat_function (void);

int
main (void)
{
  call_compat_function ();
}
EOF

gcc -fpic -shared -o symver.so -Wl,--version-script=symver.map symver.c
gcc -Wl,--rpath=. -o main main.c symver.so
######################################################################

fails with:

$ ./main
Error relocating ./symver.so: compat_function: symbol not found

The problem is the compatibility symbol (one @ instead of @@).  The
dynamic linker is supposed to ignore the difference between the two, the
default vs non-default version only matters to the link editor when
processing an undefined symbol without a symbol version.

In my case, I do not need symbol interposition and therefore can work
around this, but I wonder if there is some sort of approved compile-time
or link-time check to detect this issue.  Unfortunately, the Alpine
Linux toolchain (and part of the system) is built *with* symbol
versioning support, so this does not appear to be straightforward.

The actual application does not need to make the symbol interposable, so
I can use a hidden alias within the DSO for PLT avoidance (and more
configure checks to disable all this on targets which do not support
*that*).

Thanks,
Florian

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