Improve stack unwinder robustness.

Keep track of whether memory maps are readable.  Use the information
in try_get_word to try to avoid accidentally dereferencing an invalid
pointer within the current process.  (Note that I haven't ever
seen that happen during normal unwinding, but it pays to be
a little more careful.)

Refactored try_get_word a little to make it easier to pass it the
needed state for validation checks by way of a little memory_t struct.

Improved how the memory map for the current process is cached.  This is
important because we need up to date information about readable maps.
Use a 5 second cache expiration.

Improved the PC -> LR fallback logic in the unwinder so we can
eke out an extra frame sometimes.

Fixed a bug reading ELF program headers.  The phnum & phentsize
fields are half-words.  We were incorrectly interpreting
phnum as a whole word.

Used android_atomic_* operations carefully in the unwinder
to prevent possible memory races between the dumper and the dumpee.
This was highly unlikely (or even impossible due to the presence
of other barriers along the way) but the code is clearer now about
its invariants.

Fixed a bug in debuggerd where the pid was being passed to have
its stack dump taken instead of the tid, resulting in short
stacks because ptrace couldn't read the data if pid != tid.
Did a full sweep to ensure that we use pid / tid correctly everywhere.

Ported old code from debuggerd to rewind the program counter back
one instruction so that it points to the branch instruction itself
instead of the return address.

Change-Id: Icc4eb08320052975a4ae7f0f5f0ac9308a2d33d7

1 files changed, 2 insertions, 2 deletions

diff --git a/debuggerd/arm/machine.c b/debuggerd/arm/machine.c
index d941684..ca45c9b 100644
--- a/debuggerd/arm/machine.c
+++ b/debuggerd/arm/machine.c
@@ -87,7 +87,7 @@ static void dump_memory_and_code(int tfd, pid_t tid, bool at_fault) {
     }
 }
 
-void dump_registers(ptrace_context_t* context __attribute((unused)),
+void dump_registers(const ptrace_context_t* context __attribute((unused)),
         int tfd, pid_t tid, bool at_fault)
 {
     struct pt_regs r;
@@ -125,7 +125,7 @@ void dump_registers(ptrace_context_t* context __attribute((unused)),
 #endif
 }
 
-void dump_thread(ptrace_context_t* context, int tfd, pid_t tid, bool at_fault) {
+void dump_thread(const ptrace_context_t* context, int tfd, pid_t tid, bool at_fault) {
     dump_registers(context, tfd, tid, at_fault);
 
     dump_backtrace_and_stack(context, tfd, tid, at_fault);