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Re: [libvirt] [PATCH 2/2] Revert "Prevent more compiler optimization of mockable functions"

On Wed, Jul 12, 2017 at 11:14:16AM +0100, Daniel P. Berrange wrote:
This reverts commit e4b980c853d2114b25fa805a84ea288384416221.

When a binary links against a .a archive (as opposed to a shared library),
any symbols which are marked as 'weak' get silently dropped. As a result
when the binary later runs, those 'weak' functions have an address of
0x0 and thus crash when run.

This happened with virtlogd and virtlockd because they don't link to
libvirt.so, but instead just libvirt_util.a and libvirt_rpc.a. The
virRandomBits symbols was weak and so left out of the virtlogd &
virtlockd binaries, despite being required by virHashTable functions.

Various other binaries like libvirt_lxc, libvirt_iohelper, etc also
link directly to .a files instead of libvirt.so, so are potentially
at risk of dropping symbols leading to a later runtime crash.

This is normal linker behaviour because a weak symbol is not treated
as undefined, so nothing forces it to be pulled in from the .a You
have to force the linker to pull in weak symbols using -u$SYMNAME
which is not a practical approach.

How is this done by gnulib (or libc) when most their functions are weak
aliases anyway?  Can't we use the same approach they have?
virtlo{g,ck}d link with libgnu.la as well and there is no problem with
that, right?  So I guess this _must_ be solvable somehow, IMHO.

I'm just curious how that works.


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