In my previous fix of runtime error [1], I made a mistake of
decrementing one unconditionally, regardless of whether an extra
'syscall_nr' or 'nr' field was present in libtraceevent's tp_format.
This may cause perf trace to allocate one fewer arg_fmt entry than
needed for the accurate representation of syscall arguments.

This patch corrects the mistake by checking the presence of'syscall_nr' or
'nr', and adjusting the length of arg_fmt[] accordingly.

Signed-off-by: Howard Chu <howardchu95@gmail.com>
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Fixes: c7b87ce0dd10 ("perf trace: Fix runtime error of index out of bounds")
[1]: https://lore.kernel.org/linux-perf-users/20250122025519.361873-1-howardchu95@gmail.com/
---
Changes in v5:
- Move the link to the bottom, and keep the triple dash (suggested by Namhyung)

Changes in v4:
- Make the patch apply

Changes in v3:
- Add 'Fixes:' tag

Changes in v2:
- Simplify the code (written by Namhyung)

 tools/perf/builtin-trace.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
index a102748bd0c9..439e152186da 100644
--- a/tools/perf/builtin-trace.c
+++ b/tools/perf/builtin-trace.c
@@ -2022,9 +2022,6 @@ static int syscall__alloc_arg_fmts(struct syscall *sc, int nr_args)
 {
 	int idx;
 
-	if (nr_args == RAW_SYSCALL_ARGS_NUM && sc->fmt && sc->fmt->nr_args != 0)
-		nr_args = sc->fmt->nr_args;
-
 	sc->arg_fmt = calloc(nr_args, sizeof(*sc->arg_fmt));
 	if (sc->arg_fmt == NULL)
 		return -1;
@@ -2034,7 +2031,6 @@ static int syscall__alloc_arg_fmts(struct syscall *sc, int nr_args)
 			sc->arg_fmt[idx] = sc->fmt->arg[idx];
 	}
 
-	sc->nr_args = nr_args;
 	return 0;
 }
 
@@ -2176,14 +2172,9 @@ static int syscall__read_info(struct syscall *sc, struct trace *trace)
 		return err;
 	}
 
-	/*
-	 * The tracepoint format contains __syscall_nr field, so it's one more
-	 * than the actual number of syscall arguments.
-	 */
-	if (syscall__alloc_arg_fmts(sc, sc->tp_format->format.nr_fields - 1))
-		return -ENOMEM;
-
 	sc->args = sc->tp_format->format.fields;
+	sc->nr_args = sc->tp_format->format.nr_fields;
+
 	/*
 	 * We need to check and discard the first variable '__syscall_nr'
 	 * or 'nr' that mean the syscall number. It is needless here.
@@ -2194,6 +2185,9 @@ static int syscall__read_info(struct syscall *sc, struct trace *trace)
 		--sc->nr_args;
 	}
 
+	if (syscall__alloc_arg_fmts(sc, sc->nr_args))
+		return -ENOMEM;
+
 	sc->is_exit = !strcmp(name, "exit_group") || !strcmp(name, "exit");
 	sc->is_open = !strcmp(name, "open") || !strcmp(name, "openat");
 
-- 
2.45.2