tree d9d6aebb5812a33de9db375c6aeb0cd3ff2b6ccc
parent 0c3a6ef4ea54a179328734a45b7f7698e44ad805
author Chris Phlipot <cphlipot0@gmail.com> 1461119531 -0700
committer Arnaldo Carvalho de Melo <acme@redhat.com> 1461599357 -0300

perf script: Fix segfault when printing callchains

This fixes a bug caused by an unitialized callchain cursor. The crash
frist appeared in:

6f736735e30f ("perf evsel: Require that callchains be resolved before
calling fprintf_{sym,callchain}")

The callchain cursor is a struct that contains pointers, that when
uninitialized will cause unpredictable behavior (usually a crash)
when trying to append to the callchain.

The existing implementation has the following issues:

1. The callchain cursor used is not initialized, resulting in
	unpredictable behavior when used.
2. The cursor is declared on the stack. Even if it is properly initalized,
	the implmentation will leak memory when the function returns,
	since all the references to the callchain_nodes allocated by
	callchain_cursor_append will be lost when the cursor goes out of
	scope.
3. Storing the cursor on the stack is inefficient. Even if memory is
	properly freed when it goes out of scope, a performance penalty
	will be incurred due to reallocation of callchain nodes.
	callchain_cursor_append is designed to avoid these reallocations
	when an existing cursor is reused.

This patch fixes the crash by replacing cursor_callchain with a reference
to the global callchain_cursor which also resolves all 3 issues mentioned
above.

How to reproduce the crash:

  $ perf record --call-graph=dwarf stress -t 1 -c 1
  $ perf script > /dev/null
  Segfault

Signed-off-by: Chris Phlipot <cphlipot0@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 6f736735e30f ("perf evsel: Require that callchains be resolved before calling fprintf_{sym,callchain}")
Link: http://lkml.kernel.org/r/1461119531-2529-1-git-send-email-cphlipot0@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>