[PATCH 1/5] core: Add method to get the vmlinux BTF filename, allow overriding it via env var

Arnaldo Carvalho de Melo
2024-11-19T13:40:49Z

From: Arnaldo Carvalho de Melo <acme@redhat.com>

In systems where BTF isn't available there were reports that the
simplest pahole call, without any args, segfaults.

To have a proper test before fixing this problem, allow overriding the
/sys/kernel/btf/vmlinux filename, so that even in systems with BTF we an
point it to a invalid location, making pahole think that there is no BTF
available and thus fallback to something that currently segfaults.

Using it:

$ pahole list_head
struct list_head {
struct list_head * next; /* 0 8 */
struct list_head * prev; /* 8 8 */

/* size: 16, cachelines: 1, members: 2 */
/* last cacheline: 16 bytes */
};
$
$ PAHOLE_VMLINUX_BTF_FILENAME=foobar pahole list_head
Segmentation fault (core dumped)
$

Reported-by: Matthias Schwarzott <zzam@gentoo.org>
Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
dwarves.c | 17 +++++++++++++++--
dwarves.h | 2 ++
man-pages/pahole.1 | 4 ++++
pahole.c | 2 +-
4 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/dwarves.c b/dwarves.c
index 14ba4f0c6dd7f90f..5b51b55191177e21 100644
--- a/dwarves.c
+++ b/dwarves.c
@@ -2807,6 +2807,19 @@ static int filename__sprintf_build_id(const char *pathname, char *sbuild_id)
static int vmlinux_path__nr_entries;
static char **vmlinux_path;

+const char *vmlinux_path__btf_filename(void)
+{
+ static const char *vmlinux_btf;
+
+ if (vmlinux_btf == NULL) {
+ vmlinux_btf = getenv("PAHOLE_VMLINUX_BTF_FILENAME");
+ if (vmlinux_btf == NULL)
+ vmlinux_btf = "/sys/kernel/btf/vmlinux";
+ }
+
+ return vmlinux_btf;
+}
+
static void vmlinux_path__exit(void)
{
while (--vmlinux_path__nr_entries >= 0)
@@ -2933,7 +2946,7 @@ static int cus__load_running_kernel(struct cus *cus, struct conf_load *conf)
int err = 0;

if ((!conf || conf->format_path == NULL || strncmp(conf->format_path, "btf", 3) == 0) &&
- access("/sys/kernel/btf/vmlinux", R_OK) == 0) {
+ access(vmlinux_path__btf_filename(), R_OK) == 0) {
int loader = debugging_formats__loader("btf");
if (loader == -1)
goto try_elf;
@@ -2941,7 +2954,7 @@ static int cus__load_running_kernel(struct cus *cus, struct conf_load *conf)
if (conf && conf->conf_fprintf)
conf->conf_fprintf->has_alignment_info = debug_fmt_table[loader]->has_alignment_info;

- if (debug_fmt_table[loader]->load_file(cus, conf, "/sys/kernel/btf/vmlinux") == 0)
+ if (debug_fmt_table[loader]->load_file(cus, conf, vmlinux_path__btf_filename()) == 0)
return 0;
}
try_elf:
diff --git a/dwarves.h b/dwarves.h
index 49988f1cf9d45be8..1cb0d629f5265523 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -1628,6 +1628,8 @@ void dwarves__resolve_cacheline_size(const struct conf_load *conf, uint16_t user

const char *dwarf_tag_name(const uint32_t tag);

+const char *vmlinux_path__btf_filename(void);
+
const char *vmlinux_path__find_running_kernel(void);

struct argp_state;
diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
index bd28259b65429acc..39e7b465c64e7e2c 100644
--- a/man-pages/pahole.1
+++ b/man-pages/pahole.1
@@ -75,6 +75,10 @@ struct list_head {
};
$
.fi
+It is possible to override the /sys/kernel/btf/vmlinux file location by setting
+the PAHOLE_VMLINUX_BTF_FILENAME environment variable. This may be useful for
+testing, scripting when using a different BTF for vmlinux. Used in the pahole
+regression tests.

If BTF is not present and no file is passed, then a vmlinux that matches the
build-id for the running kernel will be looked up in the usual places,
diff --git a/pahole.c b/pahole.c
index 55d04cf82a3da683..b94cb1a979a6923d 100644
--- a/pahole.c
+++ b/pahole.c
@@ -3754,7 +3754,7 @@ try_sole_arg_as_class_names:
if (filename &&
strstarts(filename, "/sys/kernel/btf/") &&
strstr(filename, "/vmlinux") == NULL) {
- base_btf_file = "/sys/kernel/btf/vmlinux";
+ base_btf_file = vmlinux_path__btf_filename();
conf_load.base_btf = btf__parse(base_btf_file, NULL);
if (libbpf_get_error(conf_load.base_btf)) {
fprintf(stderr, "Failed to parse base BTF '%s': %ld\n",
--
2.47.0