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List:       git-commits-head
Subject:    perf record: Avoid infinite loop at buildid processing with no samples
From:       "Linux Kernel Mailing List" <linux-kernel () vger ! kernel ! org>
Date:       2015-09-27 16:54:45
Message-ID: 20150927165445.8143F661114 () gitolite ! kernel ! org
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Web:        https://git.kernel.org/torvalds/c/381c02f6d8ccad8ed574630f879c40fb59715124
Commit:     381c02f6d8ccad8ed574630f879c40fb59715124
Parent:     f73e22ab450140830005581c2c7ec389791a1b8d
Refname:    refs/heads/master
Author:     Mark Rutland <mark.rutland@arm.com>
AuthorDate: Wed Sep 16 18:18:49 2015 +0100
Committer:  Arnaldo Carvalho de Melo <acme@redhat.com>
CommitDate: Fri Sep 18 12:31:40 2015 -0300

    perf record: Avoid infinite loop at buildid processing with no samples
    
    If a session contains no events, we can get stuck in an infinite loop in
    __perf_session__process_events, with a non-zero file_size and data_offset, but
    a zero data_size.
    
    In this case, we can mmap the entirety of the file (consisting of the file and
    attribute headers), and fetch_mmaped_event will correctly refuse to read any
    (unmapped and non-existent) event headers. This causes
    __perf_session__process_events to unmap the file and retry with the exact same
    parameters, getting stuck in an infinite loop.
    
    This has been observed to result in an exit-time hang when counting
    rare/unschedulable events with perf record, and can be triggered artificially
    with the script below:
    
      ----
      #!/bin/sh
      printf "REPRO: launching perf\n";
      ./perf record -e software/config=9/ sleep 1 &
      PERF_PID=$!;
      sleep 0.002;
      kill -2 $PERF_PID;
      printf "REPRO: waiting for perf (%d) to exit...\n" "$PERF_PID";
      wait $PERF_PID;
      printf "REPRO: perf exited\n";
      ----
    
    To avoid this, have __perf_session__process_events bail out early when
    the file has no data (i.e. it has no events).
    
    Commiter note:
    
    I only managed to reproduce this when setting
    /proc/sys/kernel/kptr_restrict to '1' and changing the code to
    purposefully not process any samples and no synthesized samples, i.e.
    kptr_restrict prevents 'record' from synthesizing the kernel mmaps for
    vmlinux + modules and since it is a workload started from perf, we don't
    synthesize mmap/comm records for existing threads.
    
    Adrian Hunter managed to reproduce it in his environment tho.
    
    Signed-off-by: Mark Rutland <mark.rutland@arm.com>
    Tested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
    Tested-by: Adrian Hunter <adrian.hunter@intel.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/1442423929-12253-1-git-send-email-mark.rutland@arm.com
    Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/util/session.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index 8a4537e..fc3f7c9 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1580,7 +1580,10 @@ static int __perf_session__process_events(struct perf_session *session,
 	file_offset = page_offset;
 	head = data_offset - page_offset;
 
-	if (data_size && (data_offset + data_size < file_size))
+	if (data_size == 0)
+		goto out;
+
+	if (data_offset + data_size < file_size)
 		file_size = data_offset + data_size;
 
 	ui_progress__init(&prog, file_size, "Processing events...");
--
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