core: Add method to get the vmlinux BTF filename, allow overriding it…

… via env var

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 <[email protected]>
Reviewed-by: Alan Maguire <[email protected]>
Tested-by: Alan Maguire <[email protected]>
Tested-by: Eduard Zingerman <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/all/ZzzPUhCaew6rHwL_@x1
Link: https://lore.kernel.org/all/ZzzZpqA8ZH-6-Aj9@x1
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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)
@@ -2933,15 +2946,15 @@ 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;
 
 		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:

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;

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,

pahole.c

@@ -3754,7 +3754,7 @@ int main(int argc, char *argv[])
 		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",