Tags: egtvedt/libunwind
Tags
ppc64: Fix serious regression (many crashes in test suite)
A recent commit added code to override the unwind location for the
TOC pointer register r2:
unsigned int *inst = (unw_word_t*)c->dwarf.ip;
if (*inst == (0xE8410000 + 24)) {
// @plt call, restoring R2 from CFA+24
c->dwarf.loc[UNW_PPC64_R2] = DWARF_LOC(c->dwarf.cfa + 24, 0);
}
It is correct that such code is needed, since DWARF CFI does not
describe the unwind location for r2 on PowerPC. However, this
particular bit of code has a number of issues, which are fixed
in this patch.
First of all, the location CFA+24 is correct only for the ELFv2
ABI. In the ELFv1 ABI, the TOC location is actually CFA+40.
More problematically, attempting to read the current instruction
by just dereferencing the address in c->dwarf.ip is wrong, and
may often lead to crashes. In particular:
- During remote unwinding, this is always wrong since we're in
the wrong address space. I've used the fetch32 helper from
remote.h to use the proper access_mem under the covers.
- c->dwarf.ip may be NULL if we've reached the end-of-stack.
I've fixed this by moving the c->dwarf.ip == 0 check down
to after unwinding (instead of before), just like all other
platforms do.
- Even so, c->dwarf.ip may point to some random location if
we've gotten confused during unwinding earlier. One likely
cause for such confusion is that we did not find DWARF CFI
for some earlier frame and attempted to use the stack
backchain. The problem is that this code currently claims
all registers remain unchanges in such a frame, which is
generally wrong. In particular if the function actually
saves and modifies r31, and this is used as frame pointer
by a later frame, things will likely go quite wrong. While
it is not really possibly to completely fix this, I've at
least marked all registers as unavailable after unwinding
a frame via stack backchain.
Tested on powerpc64-linux and powerpc64le-linux. The patch fixes
about a dozen test cases that were crashing before.
Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
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