perf symbols: Save DSO loading errno to better report errors

Before, when some problem happened while trying to load the kernel
symtab, 'perf top' would show:

      ┌─Warning:───────────────────────────┐
      │The vmlinux file can't be used.     │
      │Kernel samples will not be resolved.│
      │                                    │
      │                                    │
      │Press any key...                    │
      └────────────────────────────────────┘

Now, it reports:

  # perf top --vmlinux /dev/null

      ┌─Warning:───────────────────────────────────────────┐
      │The /tmp/passwd file can't be used: Invalid ELF file│
      │Kernel samples will not be resolved.                │
      │                                                    │
      │                                                    │
      │Press any key...                                    │
      └────────────────────────────────────────────────────┘

This is possible because we now register the reason for not being able
to load the symtab in the dso->load_errno member, and provide a
dso__strerror_load() routine to format this error into a strerror like
string with a short reason for the error while loading.

That can be just forwarding the dso__strerror_load() call to
strerror_r(), or, for a separate errno range providing a custom message.

Reported-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-u5rb5uq63xqhkfb8uv2lxd5u@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

5 files changed, 96 insertions, 15 deletions

diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index 5fb8723c7128..1cb3436276d1 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -757,8 +757,10 @@ static void perf_event__process_sample(struct perf_tool *tool,
                     al.map == machine->vmlinux_maps[MAP__FUNCTION] &&
                     RB_EMPTY_ROOT(&al.map->dso->symbols[MAP__FUNCTION])) {
                         if (symbol_conf.vmlinux_name) {
-                                ui__warning("The %s file can't be used.\n%s",
-                                            symbol_conf.vmlinux_name, msg);
+                                char serr[256];
+                                dso__strerror_load(al.map->dso, serr, sizeof(serr));
+                                ui__warning("The %s file can't be used: %s\n%s",
+                                            symbol_conf.vmlinux_name, serr, msg);
                         } else {
                                 ui__warning("A vmlinux file was not found.\n%s",
                                             msg);
diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
index 0d3667f92023..fc0ddd5792a9 100644
--- a/tools/perf/util/dso.c
+++ b/tools/perf/util/dso.c
@@ -1137,3 +1137,36 @@ enum dso_type dso__type(struct dso *dso, struct machine *machine)
 
         return dso__type_fd(fd);
 }
+
+int dso__strerror_load(struct dso *dso, char *buf, size_t buflen)
+{
+        int idx, errnum = dso->load_errno;
+        /*
+         * This must have a same ordering as the enum dso_load_errno.
+         */
+        static const char *dso_load__error_str[] = {
+        "Internal tools/perf/ library error",
+        "Invalid ELF file",
+        "Can not read build id",
+        "Mismatching build id",
+        "Decompression failure",
+        };
+
+        BUG_ON(buflen == 0);
+
+        if (errnum >= 0) {
+                const char *err = strerror_r(errnum, buf, buflen);
+
+                if (err != buf)
+                        scnprintf(buf, buflen, "%s", err);
+
+                return 0;
+        }
+
+        if (errnum <  __DSO_LOAD_ERRNO__START || errnum >= __DSO_LOAD_ERRNO__END)
+                return -1;
+
+        idx = errnum - __DSO_LOAD_ERRNO__START;
+        scnprintf(buf, buflen, "%s", dso_load__error_str[idx]);
+        return 0;
+}
diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
index 88f345cc5be2..e0901b4ed8de 100644
--- a/tools/perf/util/dso.h
+++ b/tools/perf/util/dso.h
@@ -60,6 +60,31 @@ enum dso_type {
         DSO__TYPE_X32BIT,
 };
 
+enum dso_load_errno {
+        DSO_LOAD_ERRNO__SUCCESS         = 0,
+
+        /*
+         * Choose an arbitrary negative big number not to clash with standard
+         * errno since SUS requires the errno has distinct positive values.
+         * See 'Issue 6' in the link below.
+         *
+         * http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/errno.h.html
+         */
+        __DSO_LOAD_ERRNO__START         = -10000,
+
+        DSO_LOAD_ERRNO__INTERNAL_ERROR  = __DSO_LOAD_ERRNO__START,
+
+        /* for symsrc__init() */
+        DSO_LOAD_ERRNO__INVALID_ELF,
+        DSO_LOAD_ERRNO__CANNOT_READ_BUILDID,
+        DSO_LOAD_ERRNO__MISMATCHING_BUILDID,
+
+        /* for decompress_kmodule */
+        DSO_LOAD_ERRNO__DECOMPRESSION_FAILURE,
+
+        __DSO_LOAD_ERRNO__END,
+};
+
 #define DSO__SWAP(dso, type, val)                       \
 ({                                                      \
         type ____r = val;                               \
@@ -113,6 +138,7 @@ struct dso {
         enum dso_swap_type      needs_swap;
         enum dso_binary_type    symtab_type;
         enum dso_binary_type    binary_type;
+        enum dso_load_errno     load_errno;
         u8               adjust_symbols:1;
         u8               has_build_id:1;
         u8               has_srcline:1;
@@ -294,4 +320,6 @@ void dso__free_a2l(struct dso *dso);
 
 enum dso_type dso__type(struct dso *dso, struct machine *machine);
 
+int dso__strerror_load(struct dso *dso, char *buf, size_t buflen);
+
 #endif /* __PERF_DSO */
diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 78ffde9df9bf..476268c99431 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -595,10 +595,13 @@ static int decompress_kmodule(struct dso *dso, const char *name,
                 return -1;
 
         fd = mkstemp(tmpbuf);
-        if (fd < 0)
+        if (fd < 0) {
+                dso->load_errno = errno;
                 goto out;
+        }
 
         if (!decompress_to_file(m.ext, name, fd)) {
+                dso->load_errno = DSO_LOAD_ERRNO__DECOMPRESSION_FAILURE;
                 close(fd);
                 fd = -1;
         }
@@ -635,37 +638,49 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
         Elf *elf;
         int fd;
 
-        if (dso__needs_decompress(dso))
+        if (dso__needs_decompress(dso)) {
                 fd = decompress_kmodule(dso, name, type);
-        else
+                if (fd < 0)
+                        return -1;
+        } else {
                 fd = open(name, O_RDONLY);
-
-        if (fd < 0)
-                return -1;
+                if (fd < 0) {
+                        dso->load_errno = errno;
+                        return -1;
+                }
+        }
 
         elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
         if (elf == NULL) {
                 pr_debug("%s: cannot read %s ELF file.\n", __func__, name);
+                dso->load_errno = DSO_LOAD_ERRNO__INVALID_ELF;
                 goto out_close;
         }
 
         if (gelf_getehdr(elf, &ehdr) == NULL) {
+                dso->load_errno = DSO_LOAD_ERRNO__INVALID_ELF;
                 pr_debug("%s: cannot get elf header.\n", __func__);
                 goto out_elf_end;
         }
 
-        if (dso__swap_init(dso, ehdr.e_ident[EI_DATA]))
+        if (dso__swap_init(dso, ehdr.e_ident[EI_DATA])) {
+                dso->load_errno = DSO_LOAD_ERRNO__INTERNAL_ERROR;
                 goto out_elf_end;
+        }
 
         /* Always reject images with a mismatched build-id: */
         if (dso->has_build_id) {
                 u8 build_id[BUILD_ID_SIZE];
 
-                if (elf_read_build_id(elf, build_id, BUILD_ID_SIZE) < 0)
+                if (elf_read_build_id(elf, build_id, BUILD_ID_SIZE) < 0) {
+                        dso->load_errno = DSO_LOAD_ERRNO__CANNOT_READ_BUILDID;
                         goto out_elf_end;
+                }
 
-                if (!dso__build_id_equal(dso, build_id))
+                if (!dso__build_id_equal(dso, build_id)) {
+                        dso->load_errno = DSO_LOAD_ERRNO__MISMATCHING_BUILDID;
                         goto out_elf_end;
+                }
         }
 
         ss->is_64_bit = (gelf_getclass(elf) == ELFCLASS64);
@@ -701,8 +716,10 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
         }
 
         ss->name   = strdup(name);
-        if (!ss->name)
+        if (!ss->name) {
+                dso->load_errno = errno;
                 goto out_elf_end;
+        }
 
         ss->elf    = elf;
         ss->fd     = fd;
diff --git a/tools/perf/util/symbol-minimal.c b/tools/perf/util/symbol-minimal.c
index d7efb03b3f9a..fd8477cacf88 100644
--- a/tools/perf/util/symbol-minimal.c
+++ b/tools/perf/util/symbol-minimal.c
@@ -246,13 +246,12 @@ out:
         return ret;
 }
 
-int symsrc__init(struct symsrc *ss, struct dso *dso __maybe_unused,
-                 const char *name,
+int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
                  enum dso_binary_type type)
 {
         int fd = open(name, O_RDONLY);
         if (fd < 0)
-                return -1;
+                goto out_errno;
 
         ss->name = strdup(name);
         if (!ss->name)
@@ -264,6 +263,8 @@ int symsrc__init(struct symsrc *ss, struct dso *dso __maybe_unused,
         return 0;
 out_close:
         close(fd);
+out_errno:
+        dso->load_errno = errno;
         return -1;
 }