Re: [libvirt] Entering freeze for libvirt-3.5.0

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: > 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.