On Tue, Jul 04, 2017 at 05:32:03PM +0100, Daniel P. Berrange wrote:
> On Tue, Jul 04, 2017 at 12:21:21PM +0100, Daniel P. Berrange wrote:
> > On Tue, Jul 04, 2017 at 01:03:52PM +0200, Andrea Bolognani wrote:
> > > On Sat, 2017-07-01 at 14:18 +0400, Roman Bogorodskiy wrote:
> > > > > qemuxml2argvtest fails consistently in my FreeBSD guest.
> > > >
> > > > I guess that's caused by clang inlining functions that are mocked
> > > > (specifically, some numa related stuff); I think that was discussed
> > > > several times already. Anyway, it should work fine with '-O0' in CFLAGS.
> > >
> > > Well, would you look at that. It does indeed work flawlessly
> > > when compiled without optimizations! :O
> > >
> > > I'm not sure if that would be considered a reasonable
> > > compromise to get the test suite running on FreeBSD in the
> > > context of CI, though. I think it working reliably without
> > > messing with CFLAGS would be a requirement; others might
> > > disagree.
> >
> > Hmm, I thought I fixed that problem when I introduce this patch:
> >
> > commit 728cacc8abed2b8de39e7b96fa42fde6850ec23a
> > Author: Daniel P. Berrange <berrange@xxxxxxxxxx>
> > Date: Fri Apr 7 15:07:49 2017 +0100
> >
> > annotate all mocked functions with noinline
> >
> >
> > This made us annotate all mocked functions with noinline, which was
> > sufficient to make CLang builds pass tests on Ubuntu VMs.
>
> So I did some debugging and this is wierder than I can imagine possible.
>
> I put a printf statement in virNumaGetMaxNode in qemuxml2argvmock.c
> to print out the return value.
>
> I also put a printf statement in virNumaNodeIsAvailable in virnuma.c
> (in the non-NUMACTL conditional block), that prints out the return
> value received from virNumaGetMaxNode
>
> The first printf displays 7, while the second printf displays -1
>
> So we're definitely calling our mock override, but the return
> value is getting mangled when seen by the caller, which is a
> giant wtf to me.
I wrote an isolated test case
$ cat lib.h
int get_max(void) __attribute__((noinline));
int is_ok(int i);
$ cat lib.c
#include <stdio.h>
#include "lib.h"
int get_max(void)
{
fprintf(stderr, "In original max, returning 3\n");
return 3;
}
int is_ok(int i)
{
int max = get_max();
fprintf(stderr, "Received max %d\n", max);
return i > 0 && i <= max;
}
$ cat mock.c
#include <stdio.h>
#include "lib.h"
int get_max(void)
{
fprintf(stderr, "In mock max, returning 7\n");
return 7;
}
$ cat run.c
#include <stdio.h>
#include "lib.h"
int main(int argc, char **argv)
{
fprintf(stderr, "Is 5 in range ? %d\n", is_ok(5));
}
$ clang -O2 -g -Wall -shared -o libdemo.so -fPIC lib.c
$ clang -O2 -g -Wall -shared -o mock.so -fPIC mock.c
$ clang -O2 -Wall -o run run.c -L. -ldemo
$ ./run
In original max, returning 3
Received max 3
Is 5 in range ? 0
$ LD_PRELOAD=mock.so ./run
In mock max, returning 7
Received max 3
Is 5 in range ? 0
$ clang -O0 -g -Wall -shared -o libdemo.so -fPIC lib.c
$ LD_PRELOAD=mock.so ./run
In mock max, returning 7
Received max 7
Is 5 in range ? 1
So clang is definitely *not* inlining the function, *is* running out mock
function, but none the less getting the return value from the original
function.
Turning on optimizer debugging i see
$ clang -O2 -g -Wall -shared -o libdemo.so -Rpass=.* -fPIC lib.c
lib.c:7:3: remark: marked this call a tail call candidate [-Rpass=tailcallelim]
fprintf(stderr, "In original max, returning 3\n");
^
lib.c:11:15: remark: marked this call a tail call candidate [-Rpass=tailcallelim]
int is_ok(int i)
^
lib.c:13:13: remark: marked this call a tail call candidate [-Rpass=tailcallelim]
int max = get_max();
^
lib.c:13:7: remark: marked this call a tail call candidate [-Rpass=tailcallelim]
int max = get_max();
^
lib.c:14:3: remark: marked this call a tail call candidate [-Rpass=tailcallelim]
fprintf(stderr, "Received max %d\n", max);
^
so, I'm thinking this problem is a result of tail call optimization making
an assumption that is violated when mocking the function.
I'm unclear how to prevent tail call optimization without the big hammer
of -O0
Regards,
Daniel
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