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



On Thu, Jul 13, 2017 at 12:28:41PM +0100, Daniel P. Berrange wrote:
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


Sure, ACK to that.

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