linux-kernel - [PATCH] perf: top: fix a segfault when kernel map is restricted.

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Message-ID: <1429918529-225417-1-git-send-email-wangnan0@huawei.com>
Date:	Fri, 24 Apr 2015 23:35:29 +0000
From:	Wang Nan <wangnan0@...wei.com>
To:	<a.p.zijlstra@...llo.nl>, <paulus@...ba.org0>, <acme@...nel.org>,
	<dsahern@...il.com>, <jolsa@...nel.org>
CC:	<mingo@...hat.com>, <lizefan@...wei.com>,
	<linux-kernel@...r.kernel.org>
Subject: [PATCH] perf: top: fix a segfault when kernel map is restricted.

Perf top raise a warning if a kernel sample is collected but kernel map
is restricted. The warning message needs to dereference al.map->dso...
However, previous perf_event__preprocess_sample() doesn't always
guarantee al.map != NULL, for example, when kernel map is restricted.

This patch validates al.map before dereferencing, avoid the segfault.

Before this patch:

 $ cat /proc/sys/kernel/kptr_restrict
 1
 $ perf top -p  120183
 perf: Segmentation fault
 -------- backtrace --------
 /path/to/perf[0x509868]
 /lib64/libc.so.6(+0x3545f)[0x7f9a1540045f]
 /path/to/perf[0x448820]
 /path/to/perf(cmd_top+0xe3c)[0x44a5dc]
 /path/to/perf[0x4766a2]
 /path/to/perf(main+0x5f5)[0x42e545]
 /lib64/libc.so.6(__libc_start_main+0xf4)[0x7f9a153ecbd4]
 /path/to/perf[0x42e674]

And gdb call trace:

 Program received signal SIGSEGV, Segmentation fault.
 perf_event__process_sample (machine=0xa44030, sample=0x7fffffffa4c0, evsel=0xa43b00, event=0x7ffff41c3000, tool=0x7fffffffa8a0)
    at builtin-top.c:736
 736				  !RB_EMPTY_ROOT(&al.map->dso->symbols[MAP__FUNCTION]) ?
 (gdb) bt
 #0  perf_event__process_sample (machine=0xa44030, sample=0x7fffffffa4c0, evsel=0xa43b00, event=0x7ffff41c3000, tool=0x7fffffffa8a0)
     at builtin-top.c:736
 #1  perf_top__mmap_read_idx (top=top@...ry=0x7fffffffa8a0, idx=idx@...ry=0) at builtin-top.c:855
 #2  0x000000000044a5dd in perf_top__mmap_read (top=0x7fffffffa8a0) at builtin-top.c:872
 #3  __cmd_top (top=0x7fffffffa8a0) at builtin-top.c:997
 #4  cmd_top (argc=<optimized out>, argv=<optimized out>, prefix=<optimized out>) at builtin-top.c:1267
 #5  0x00000000004766a3 in run_builtin (p=p@...ry=0x8a6ce8 <commands+264>, argc=argc@...ry=3, argv=argv@...ry=0x7fffffffdf70)
      at perf.c:371
 #6  0x000000000042e546 in handle_internal_command (argv=0x7fffffffdf70, argc=3) at perf.c:430
 #7  run_argv (argv=0x7fffffffdcf0, argcp=0x7fffffffdcfc) at perf.c:474
 #8  main (argc=3, argv=0x7fffffffdf70) at perf.c:589
 (gdb)

Signed-off-by: Wang Nan <wangnan0@...wei.com>
---
 tools/perf/builtin-top.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index 1cb3436..6a4d5d4 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -733,7 +733,7 @@ static void perf_event__process_sample(struct perf_tool *tool,
 "Kernel address maps (/proc/{kallsyms,modules}) are restricted.\n\n"
 "Check /proc/sys/kernel/kptr_restrict.\n\n"
 "Kernel%s samples will not be resolved.\n",
-			  !RB_EMPTY_ROOT(&al.map->dso->symbols[MAP__FUNCTION]) ?
+			  al.map && !RB_EMPTY_ROOT(&al.map->dso->symbols[MAP__FUNCTION]) ?
 			  " modules" : "");
 		if (use_browser <= 0)
 			sleep(5);
-- 
1.8.3.4

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