PR25882, .gnu.attributes are not checked for shared libraries

[Alan Modra]

On Fri, May 01, 2020 at 04:48:38PM -0700, H.J. Lu wrote:
> On Fri, May 1, 2020 at 4:22 PM Joseph Myers <joseph@codesourcery.com> wrote:
> >
> > This binutils patch breaks glibc "make check" for big-endian powerpc
> > (32-bit and 64-bit).
> >
> > https://sourceware.org/pipermail/libc-testresults/2020q2/006097.html
> >
> > Errors of the form:
> >
> > /scratch/jmyers/glibc-bot/install/compilers/powerpc64-linux-gnu/lib/gcc/powerpc64-glibc-linux-gnu/11.0.0/../../../../powerpc64-glibc-linux-gnu/bin/ld: /scratch/jmyers/glibc-bot/build/glibcs/powerpc64-linux-gnu/glibc/math/test-nldbl-redirect.o uses 64-bit long double, /scratch/jmyers/glibc-bot/build/glibcs/powerpc64-linux-gnu/glibc/math/libm.so.6 uses 128-bit long double

Hmm, is the Tag_GNU_Power_ABI_FP claim of ibm long double strictly
correct?  Doesn't libm also support 64-bit long double too?  In which
case libm ought to be silent about long double support.

That of course can be accomplished by compiling all the object files
going into libm.so that provide long double functions (or call such
functions) with -mno-gnu-attributes.  Another method with slightly
more finesse is to compile all files with the default -mgnu-attributes
but link with --no-warn-mismatch.  That way the linker will not give
an error and also not emit the particular Tag with a conflict.  (This
method assumes that assembly files emit the correct tags.)

> > I don't know whether this is a problem with binutils, or it indicates that
> > BE powerpc glibc needs to use -mno-gnu-attribute on some files like LE
> > does to avoid such errors.
> 
> It is odd to check .gnu.attributes on shared libraries since shared libraries
> used at link-time can be very different from run-time.

You can say the same about many properties of shared libraries.
ld.gold on both arm and powerpc does check these attributes for shared
libraries.  And it's necessary to use ld.gold to link split-stack and
non-split-stack object files if you want to ensure the non-split-stack
functions are called with something like the normal stack allocation.
If you use ld.bfd then non-split-stack functions might be called with
very small stacks.  So arm and powerpc have a history of checking
shared library .gnu.attributes.

>  Checking .gnu.attributes
> on shared libraries gives you very little.

Checking .gnu.attributes protects the user from linking against a
shared library that say, provides ibm long double math function when
the user is compiling for ieee long double.  That was the idea behind
the binutils patch.  There are of course some negatives, chief of
which is that .gnu.attributes is nowhere near fine enough grained to
work well with libraries that provide multiple areas of support.  You
may hit a link error regarding functions you do not use in your app.

-- 
Alan Modra
Australia Development Lab, IBM