[PATCH v3 05/16] perf trace: Do not set unaugmented BPF program on sys_exit map

[Ian Rogers]

In trace__init_syscalls_bpf_prog_array_maps(), the BPF program array map
for sys_exit (syscalls_sys_exit) was populated with the result of
trace__bpf_prog_sys_exit_fd().

When a syscall had no specific exit augmenter,
trace__find_syscall_bpf_prog() fell back to unaugmented_prog
(syscall_unaugmented). However, syscall_unaugmented is a sys_enter
program that outputs enter arguments to __augmented_syscalls__.

As a consequence, when an unaugmented syscall exited, sys_exit
tail-called syscall_unaugmented, which interpreted the exit arguments as
enter arguments and emitted a duplicate, corrupt sys_enter event into
__augmented_syscalls__ right as the syscall completed.

Fix this by:
1. Returning NULL from trace__find_syscall_bpf_prog() when looking up exit
   augmenters and none is found.
2. Returning -1 from trace__bpf_prog_sys_exit_fd() when no exit program
   is present.
3. Only updating map_exit_fd when prog_fd >= 0.
4. Clearing err = 0 when trace__bpf_sys_enter_beauty_map() returns
   non-zero (indicating the syscall has no augmentable pointer arguments)
   before continuing the loop, so a trailing run of such syscalls (e.g.
   'perf trace -e close') does not leave err non-zero on return and abort
   the session.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 tools/perf/builtin-trace.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
index 2fbe1bca511c..2c62d38b19ac 100644
--- a/tools/perf/builtin-trace.c
+++ b/tools/perf/builtin-trace.c
@@ -4123,7 +4123,12 @@ static struct bpf_program *trace__find_syscall_bpf_prog(struct trace *trace __ma
 	pr_debug("Couldn't find BPF prog \"%s\" to associate with syscalls:sys_%s_%s, not augmenting it\n",
 		 prog_name, type, sc->name);
 out_unaugmented:
-	return unaugmented_prog;
+	/*
+	 * Do not set unaugmented_prog for exit: syscall_unaugmented is a
+	 * sys_enter program that outputs enter arguments. Exit without a
+	 * specialized return augmenter returns 1 directly from sys_exit.
+	 */
+	return !strcmp(type, "exit") ? NULL : unaugmented_prog;
 }
 
 static void trace__init_syscall_bpf_progs(struct trace *trace, int e_machine, int id)
@@ -4146,7 +4151,7 @@ static int trace__bpf_prog_sys_enter_fd(struct trace *trace, int e_machine, int
 static int trace__bpf_prog_sys_exit_fd(struct trace *trace, int e_machine, int id)
 {
 	struct syscall *sc = trace__syscall_info(trace, NULL, e_machine, id);
-	return sc ? bpf_program__fd(sc->bpf_prog.sys_exit) : bpf_program__fd(unaugmented_prog);
+	return sc && sc->bpf_prog.sys_exit ? bpf_program__fd(sc->bpf_prog.sys_exit) : -1;
 }
 
 static int trace__bpf_sys_enter_beauty_map(struct trace *trace, int e_machine, int key, unsigned int *beauty_array)
@@ -4401,16 +4406,27 @@ static int trace__init_syscalls_bpf_prog_array_maps(struct trace *trace, int e_m
 		err = bpf_map_update_elem(map_enter_fd, &key, &prog_fd, BPF_ANY);
 		if (err)
 			break;
+		/* Only update the exit prog array map if an exit augmenter exists */
 		prog_fd = trace__bpf_prog_sys_exit_fd(trace, e_machine, key);
-		err = bpf_map_update_elem(map_exit_fd, &key, &prog_fd, BPF_ANY);
-		if (err)
-			break;
+		if (prog_fd >= 0) {
+			err = bpf_map_update_elem(map_exit_fd, &key, &prog_fd, BPF_ANY);
+			if (err)
+				break;
+		}
 
 		/* use beauty_map to tell BPF how many bytes to collect, set beauty_map's value here */
 		memset(beauty_array, 0, sizeof(beauty_array));
 		err = trace__bpf_sys_enter_beauty_map(trace, e_machine, key, (unsigned int *)beauty_array);
-		if (err)
+		if (err) {
+			/*
+			 * Not a failure: the syscall just has no augmentable
+			 * arguments. Clear err, or a trailing run of such
+			 * syscalls, e.g. all of them for 'perf trace -e close',
+			 * would leave it set on return and abort the session.
+			 */
+			err = 0;
 			continue;
+		}
 		err = bpf_map_update_elem(beauty_map_fd, &key, beauty_array, BPF_ANY);
 		if (err)
 			break;
-- 
2.55.0.1082.g2b9226bbc0-goog