[PATCH 3/3] perf record: Fix s390 missing module symbol and warning for non-root users

[Thomas Richter]

Command 'perf record' and 'perf report' on a system without kernel
debuginfo packages uses /proc/kallsyms and /proc/modules to find
addresses for kernel and module symbols. On x86 this works for root
and non-root users.

On s390, when invoked as non-root user, many of the following warnings
are shown and module symbols are missing:

    proc/{kallsyms,modules} inconsistency while looking for
        "[sha1_s390]" module!

Command 'perf record' creates a list of module start addresses by
parsing the output of /proc/modules and creates a PERF_RECORD_MMAP
record for the kernel and each module. The following function call
sequence is executed:

  machine__create_kernel_maps
    machine__create_module
      modules__parse
        machine__create_module --> for each line in /proc/modules
          arch__fix_module_text_start

Function arch__fix_module_text_start() is s390 specific. It opens
file /sys/module/<name>/sections/.text to extract the module's .text
section start address. On s390 the module loader prepends a header
before the first section, whereas on x86 the module's text section
address is identical the the module's load address.

However module section files are root readable only. For non-root the
read operation fails and machine__create_module() returns an error.
Command perf record does not generate any PERF_RECORD_MMAP record
for loaded modules. Later command perf report complains about missing
module maps.

To fix this function arch__fix_module_text_start() always returns
success. For root users there is no change, for non-root users
the module's load address is used as module's text start address
(the prepended header then counts as part of the text section).

This enable non-root users to use module symbols and avoid the
warning when perf report is executed.

Output before:
 [tmricht@m83lp54 perf]$ ./perf report -D | fgrep MMAP
 0 0x168 [0x50]: PERF_RECORD_MMAP ... x [kernel.kallsyms]_text

Output after:
 [tmricht@m83lp54 perf]$ ./perf report -D | fgrep MMAP
 0 0x168 [0x50]: PERF_RECORD_MMAP ... x [kernel.kallsyms]_text
 0 0x1b8 [0x98]: PERF_RECORD_MMAP ... x /lib/modules/.../autofs4.ko.xz
 0 0x250 [0xa8]: PERF_RECORD_MMAP ... x /lib/modules/.../sha_common.ko.xz
 0 0x2f8 [0x98]: PERF_RECORD_MMAP ... x /lib/modules/.../des_generic.ko.xz

Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
Reviewed-by: Hendrik Brueckner <brueckner@xxxxxxxxxxxxx>
---
 tools/perf/arch/s390/util/machine.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/tools/perf/arch/s390/util/machine.c b/tools/perf/arch/s390/util/machine.c
index 0b2054007314..a19690a17291 100644
--- a/tools/perf/arch/s390/util/machine.c
+++ b/tools/perf/arch/s390/util/machine.c
@@ -5,16 +5,19 @@
 #include "util.h"
 #include "machine.h"
 #include "api/fs/fs.h"
+#include "debug.h"
 
 int arch__fix_module_text_start(u64 *start, const char *name)
 {
+	u64 m_start = *start;
 	char path[PATH_MAX];
 
 	snprintf(path, PATH_MAX, "module/%.*s/sections/.text",
 				(int)strlen(name) - 2, name + 1);
-
-	if (sysfs__read_ull(path, (unsigned long long *)start) < 0)
-		return -1;
+	if (sysfs__read_ull(path, (unsigned long long *)start) < 0) {
+		pr_debug2("Using module %s start:%#lx\n", path, m_start);
+		*start = m_start;
+	}
 
 	return 0;
 }
-- 
2.19.1