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eh_elf: tolerate slightly out-of-bound mem access

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Recover from slightly out-of-bounds wrt. %rsp memory accesses.

When we're unwinding the first frame of a chain, and we start from, eg.
a `pop %rbx` at the end of the function, the DWARF can be out of sync
and still state that eg. `%rbx` is saved at something that simplifies to
`%rsp-8`. Which might not be accessibe through the `access_mem`
abstraction, eg. if we're running perf and it didn't capture the stack
below `%rsp`
This commit is contained in:
Théophile Bastian 2019-06-09 03:42:28 +02:00
parent cc2d07f550
commit bba251cf13
1 changed files with 20 additions and 1 deletions
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21
src/eh_elf/eh_elf.c
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@ -39,6 +39,7 @@ void eh_elf_clear() {
static struct {
struct cursor* cursor;
int last_rc;
uintptr_t cur_rsp;
} _fetch_state;
static uintptr_t fetchw_here(uintptr_t addr) {
@ -51,7 +52,24 @@ static uintptr_t fetchw_here(uintptr_t addr) {
_fetch_state.cursor->dwarf.as_arg);
if(rv != 0) {
Debug(1, "dwarf_get error %d\n", rv);
/*
Recover from slightly out-of-sync DWARF
====
When we're unwinding the first frame of a chain, and we start from,
eg. a `pop %rbx` at the end of the function, the DWARF can be out
of sync and still state that eg. `%rbx` is saved at something that
simplifies to `%rsp-8`. Which might not be accessibe through the
`access_mem` abstraction, eg. if we're running perf and it didn't
capture the stack below `%rsp`
*/
if(_fetch_state.cur_rsp - addr < 128) {
Debug(3, "dwarf_get warning: tried to access %lX (%lX below rsp)\n",
addr, _fetch_state.cur_rsp - addr);
return 0; // hope that nothing bad will happen.
}
Debug(1, "dwarf_get error %d (addr %lX, sp %lX)\n",
rv, addr, _fetch_state.cur_rsp);
_fetch_state.last_rc = rv;
}
@ -112,6 +130,7 @@ int eh_elf_step_cursor(struct cursor *cursor) {
// Set _fetch_state before passing fetchw_here
_fetch_state.cursor = cursor;
_fetch_state.last_rc = 0;
_fetch_state.cur_rsp = cursor->dwarf.cfa;
Debug(4, "Unwinding in mmap entry %s at position 0x%lx (sp=%016lx)\n",
mmap_entry->object_name,