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



On Wed, Jul 12, 2017 at 01:49:04PM +0200, Martin Kletzander wrote:
> On Wed, Jul 12, 2017 at 01:10:08PM +0200, Martin Kletzander wrote:
> > 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.
> > 
> > Martin
> 
> I guess we would have to do something like the following, but for every
> function.
> 
> diff --git i/src/util/virrandom.c w/src/util/virrandom.c
> index 41daa404b273..3d9fe7f85d97 100644
> --- i/src/util/virrandom.c
> +++ w/src/util/virrandom.c
> @@ -99,7 +99,7 @@ VIR_ONCE_GLOBAL_INIT(virRandom)
>  *
>  * Return: a random number with @nbits entropy
>  */
> -uint64_t virRandomBits(int nbits)
> +static uint64_t __virRandomBits(int nbits)
> {
>     uint64_t ret = 0;
>     int32_t bits;
> @@ -125,6 +125,7 @@ uint64_t virRandomBits(int nbits)
>     virMutexUnlock(&randomLock);
>     return ret;
> }
> +uint64_t virRandomBits(int nbits) ATTRIBUTE_MOCKABLE __attribute__((alias("__virRandomBits")));
> 
> 
> /**
> diff --git i/src/util/virrandom.h w/src/util/virrandom.h
> index 990a456addf7..abda95aef506 100644
> --- i/src/util/virrandom.h
> +++ w/src/util/virrandom.h
> @@ -24,7 +24,7 @@
> 
> # include "internal.h"
> 
> -uint64_t virRandomBits(int nbits) ATTRIBUTE_MOCKABLE;
> +uint64_t virRandomBits(int nbits);
> double virRandom(void);
> uint32_t virRandomInt(uint32_t max);
> int virRandomBytes(unsigned char *buf, size_t buflen)
> --
> 
> And of course that could be macrofied so that ATTRIBUTE_MOCKABLE takes
> function or something, etc.
> 
> I like this more than reverting the patches.

FYI, I intend to push these revert patches, so that virtlogd stops
crashing to unblock other devs/users, while we focus on writing &
reviewing the new approach we've discussed

Regards,
Daniel
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