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>
This patch adds support for the powerpc64le-linux platform. It consists
of two main features:
- Support little-endian byte order
This is done via a "big_endian" member of struct unw_addr_space,
which is evaluated by common code via the dwarf_is_big_endian
macro, and also in endian-aware code in unw_is_signal_frame.
- Support the ELFv2 ABI
This is done via an "abi" member of struct unw_addr_space. This
is currently only needed in tdep_get_func_addr, since the ELFv2
ABI does not use function descriptors.
Both new members are initialized in unw_create_addr_space and
ppc64_local_addr_space_init, following the mips precedent.
Since ppc32 and ppc64 now no longer share the unw_create_addr_space
implementation, the file is duplicated from the ppc directory into
ppc32/ppc64.
Tested on powerpc64-linux and powerpc64le-linux. Support on LE
seems to be as good as existing BE support; I have not attempted to
fix the existing shortcomings of PPC support that already cause a
number to tests to fail due to unimplemented features.
Signed-off-by: Ulrich Weigand <uweigand@de.ibm.com>
ppc32/Gstep.c:116: warning: comparison between pointer and integer
ppc32/Gstep.c:116: warning: comparison with string literal results in unspecified behavior
ppc32/Gstep.c:116: warning: initialization makes integer from pointer without a cast
ppc32/Gstep.c:116: warning: passing argument 2 of 'fprintf' makes pointer from integer without a cast
/usr/powerpc-linux-gnu/include/stdio.h:333: note: expected 'const char *__restrict__' but argument is of type 'int'
Adds new function to perform a pure stack walk without unwinding,
functionally similar to backtrace() but accelerated by an address
attribute cache the caller maintains across calls.
the instruction after the call for a normal frame. libunwind uses
IP-1 to lookup unwind information. However, this is not necessary for
interrupted frames such as signal frames (or interrupt frames) in
the kernel context.
This patch handles both cases correctly.
Based on work by Mark Wielaard <mwielaard@redhat.com>
Greetings,
We use libunwind just for stack traces (I suspect many others do as well).
The use pattern is:
GetStackTrace(void** result, int max_depth)
{
...
unw_getcontext(&uc);
unw_init_local(&cursor, &uc);
while (n < max_depth) {
if (unw_get_reg(&cursor, UNW_REG_IP, (unw_word_t *) &ip) < 0) {
break;
}
result[n++] = ip;
if (unw_step(&cursor) <= 0) {
break;
}
}
Given this usage, it is quite convenient for us to block signals (or
prevent signal handlers from re-entering libunwind by other means) at the
"top level", which makes most of the sigprocmask calls performed by
libunwind itself unneccessary.
The second patch in this series adds a configure option which removes most
of the sigprocmask calls.
Attached patch is a preliminary for it -- consolidating all of the
"sigprocmask; mutex_lock;" sequences into lock_acquire and "mutex_unlock;
sigprocmask;" sequences into lock_release.
Thanks,
--
Paul Pluzhnikov
commit 402d15b123d54a7669db7cf17a76dd315094e472
Author: Paul Pluzhnikov <ppluzhnikov@google.com>
Date: Mon Sep 21 10:18:28 2009 -0700
Replace "sigprocmask + mutext_lock" with a single lock_acquire.
Likewise, replace "mutext_unlock + sigprocmask" with lock_release.
On some systems executable stacks are denied. Since libunwind and the
tests don't actually need executable stacks this patch marks all
assembly files as not needing it.
The original patch comes from frysk:
2007-04-05 Jan Kratochvil <jan.kratochvil@redhat.com>
* src/hppa/getcontext.S, src/hppa/setcontext.S, src/hppa/siglongjmp.S,
src/ia64/Ginstall_cursor.S, src/ia64/Linstall_cursor.S,
src/ia64/dyn_info_list.S, src/ia64/getcontext.S, src/ia64/longjmp.S,
src/ia64/setjmp.S, src/ia64/siglongjmp.S, src/ia64/sigsetjmp.S,
src/ppc64/longjmp.S, src/ppc64/siglongjmp.S, src/x86/longjmp.S,
src/x86/siglongjmp.S, src/x86_64/longjmp.S, src/x86_64/setcontext.S,
src/x86_64/siglongjmp.S: Stack should be non-executable, for SELinux.
I added a couple more markers for new files in current libunwind.
Before this patch you would get the following on selinux enabled
systems without allow_exec_stack: error while loading shared
libraries:
libunwind.so.7: cannot enable executable stack as shared object
requires: Permission denied
After the patch that error disappears and all test results are similar
to the results on systems without executable stack protection.
routine and add address-space argument. This is needed because on
PPC64, a the function-name symbol refers to a function descriptor
(unlike, for example, on ia64, where the @fptr() operator is needed to
refer to a function descriptor). Thus, in order to look up the name
of a function, we need to dereference the function descriptor. To
make matters more "interesting", the function descriptors are normally
resolved by the dynamic linker, so we can't get their values from the
ELF file. Instead, we have to read them from the running image, hence
the need for the address-space argument.
