On Dienstag, 2. Februar 2021 06:19:42 CET Roman Bolshakov wrote:
> 'make check' of libtasn1 doesn't succeed on x86_64 either.
>
> After a session of debugging I believe there's an issue with Clang 12.
> Here's a test program (it reproduces unexpected ASN1_VALUE_NOT_VALID
> from _asn1_time_der() in libtasn1):
>
> #include <stdio.h>
>
> static int func2(char *foo) {
> fprintf(stderr, "%s:%d foo: %p\n", __func__, __LINE__, foo);
> if (foo == NULL) {
> fprintf(stderr, "%s:%d foo: %p\n", __func__, __LINE__, foo);
> return 1;
> }
> return 0;
> }
>
> int func1(char *foo) {
> int counter = 0;
> if (fprintf(stderr, "IO\n") > 0)
> counter += 10;
> fprintf(stderr, "%s:%d foo: %p counter %d\n", __func__, __LINE__,
> foo, counter); if(!func2(foo + counter)) {
> fprintf(stderr, "good\n");
> return 0;
> } else {
> fprintf(stderr, "broken\n");
> return 1;
> }
> }
>
> int main() {
> char *foo = NULL;
> return func1(foo);
> }
>
>
> What return value would you expect from the program?
>
> If the program is compiled with -O0/O1 it returns zero exit code.
> Here's the output:
> IO
> func1:16 foo: 0x0 counter 10
> func2:4 foo: 0xa
> good
>
> If it is compiled with -O2 it returns 1:
> IO
> func1:16 foo: 0x0 counter 10
> func2:4 foo: 0xa
> func2:6 foo: 0x0
> broken
>
> That happens because clang uses register behind foo from func1 (it has zero
> pointer) inside inlined func2 (it should have non zero pointer).
>
> So, immediate workaround would be to downgrade optimization level of
> libtasn1 to -O1 in homebrew.
Hu, confirmed.
clang 12.0.0 on x86_64 Mac fails on that demo with -O2,-O3,-Os, but works with
-O0,-O1.
clang 11.0.3 in contrast works with any optimization level.
It only fails BTW if that test uses exactly a NULL pointer, any other memory
address (e.g. just (void*)1) works:
#include <stdio.h>
#define FLOOR_VALUE ((void*)1)
static int func2(char *foo) {
fprintf(stderr, "%s:%d foo: %p\n", __func__, __LINE__, foo);
if (foo == FLOOR_VALUE) {
fprintf(stderr, "%s:%d foo: %p\n", __func__, __LINE__, foo);
return 1;
}
return 0;
}
int func1(char *foo) {
int counter = 0;
if (fprintf(stderr, "IO\n") > 0)
counter += 1;
fprintf(stderr, "%s:%d foo: %p counter %d\n", __func__, __LINE__, foo,
counter);
if(!func2(foo + counter)) {
fprintf(stderr, "good\n");
return 0;
} else {
fprintf(stderr, "broken\n");
return 1;
}
}
int main() {
char *foo = FLOOR_VALUE;
return func1(foo);
}
Maybe that's some sort of new security feature in clang 12, in the sense of
something like this:
VeryLargeStruct *p = NULL;
p->farMember = value;
to segfault always reliably and exactly with address zero, instead of pure
luck as of NULL + veryLargeSize.
> I've submitted the issue to Apple bugtracker:
> FB8986815
>
> Best regards,
> Roman
They could argue that operating on a NULL pointer is undefined behaviour.
Best regards,
Christian Schoenebeck
