[PATCH v4 1/6] perf dwarf-aux: Fix libdw segmentation fault in cu_walk_functions_at

[Ian Rogers <irogers@google.com>]

A segmentation fault was observed in `libdw` when running `perf kmem`
with `--page stat` on some workloads. The crash occurred deep inside
`libdw` (specifically in `dwarf_child` and `dwarf_diename`) when
processing DWARF information.

The root cause was improper error handling of `dwarf_getfuncs` in
`die_find_realfunc` and `die_find_tailfunc`.

`dwarf_getfuncs` returns:
 - `0` on success (when all functions have been processed).
 - A positive offset if the callback aborts early (e.g., via
   `DWARF_CB_ABORT` when a match is found).
 - `-1` on error.

The original code used `if (!dwarf_getfuncs(...)) return NULL;`. On
error (`-1`), `!-1` evaluates to `0` (false), bypassing the error
check. Execution then proceeded as if a match was found, returning
uninitialized stack memory (`die_mem`) to the caller
(`cu_walk_functions_at`). When `cu_walk_functions_at` passed this
uninitialized memory to `libdw` via `dwarf_diename`, it caused a
segmentation fault.

Fix this by correcting the error check to `if (dwarf_getfuncs(...) <= 0)`.

Fixes: e0d153c69040 ("perf-probe: Move dwarf library routines to dwarf-aux.{c, h}")
Fixes: d4c537e6bf86 ("perf probe: Ignore tail calls to probed functions")
Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/dwarf-aux.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
index 92db2fccc788..109a166a6d19 100644
--- a/tools/perf/util/dwarf-aux.c
+++ b/tools/perf/util/dwarf-aux.c
@@ -171,7 +171,6 @@ int cu_walk_functions_at(Dwarf_Die *cu_die, Dwarf_Addr addr,
 	}
 
 	return ret;
-
 }
 
 /**
@@ -620,7 +619,7 @@ Dwarf_Die *die_find_tailfunc(Dwarf_Die *cu_die, Dwarf_Addr addr,
 	ad.addr = addr;
 	ad.die_mem = die_mem;
 	/* dwarf_getscopes can't find subprogram. */
-	if (!dwarf_getfuncs(cu_die, __die_search_func_tail_cb, &ad, 0))
+	if (dwarf_getfuncs(cu_die, __die_search_func_tail_cb, &ad, 0) <= 0)
 		return NULL;
 	else
 		return die_mem;
@@ -659,7 +658,7 @@ Dwarf_Die *die_find_realfunc(Dwarf_Die *cu_die, Dwarf_Addr addr,
 	ad.addr = addr;
 	ad.die_mem = die_mem;
 	/* dwarf_getscopes can't find subprogram. */
-	if (!dwarf_getfuncs(cu_die, __die_search_func_cb, &ad, 0))
+	if (dwarf_getfuncs(cu_die, __die_search_func_cb, &ad, 0) <= 0)
 		return NULL;
 	else
 		return die_mem;
-- 
2.54.0.545.g6539524ca2-goog