[PATCH] Fix tst-cancel17/tst-cancelx17, which sometimes segfaults while exiting.

[Stefan Liebler <stli at linux dot vnet dot ibm dot com>]

Hi,

The testcase tst-cancel[x]17 ends sometimes with a segmentation fault.
This happens in one of 10000 cases. Then the real testcase has already
exited with success and returned from do_test(). The segmentation fault
occurs after returning from main in _dl_fini().

In those cases, the aio_read(&a) was not canceled, because the read
request was already in progress. In the meanwhile aio_write(ap) wrote
something to the pipe and the read request is able to read the
requested byte.
The read request hasn't finished before returning from do_test().
After it finishes, it writes the return value and error code from the
read syscall to the struct aiocb a, which lays on the stack of do_test.
The stack of the subsequent function call of _dl_fini or _dl_sort_fini,
which is inlined in _dl_fini, is corrupted.

In case of S390, it reads a zero and decrements it by 1:
unsigned int k = nmaps - 1;
struct link_map **runp = maps[k]->l_initfini;
The subsequent load from unmapped memory leads to the segmentation
fault.

The stack corruption also happens on other architectures.
I saw them e.g. on x86 and ppc, too.

This patch adds an aio_suspend call to ensure, that the read request
is finished before returning from do_test().

Okay to commit?

Bye
Stefan

ChangeLog:

	* nptl/tst-cancel17.c (do_test):
	Wait for finishing aio_read(&a).

commit fd27163dc65df76132946c3eca29dfb82fea2fcc
Author: Stefan Liebler <stli@linux.vnet.ibm.com>
Date:   Fri May 13 15:10:30 2016 -0400

    Fix tst-cancel17/tst-cancelx17, which sometimes segfaults while exiting.
    
    The testcase tst-cancel[x]17 ends sometimes with a segmentation fault.
    This happens in one of 10000 cases. Then the real testcase has already
    exited with success and returned from do_test(). The segmentation fault
    occurs after returning from main in _dl_fini().
    
    In those cases, the aio_read(&a) was not canceled, because the read
    request was already in progress. In the meanwhile aio_write(ap) wrote
    something to the pipe and the read request is able to read the
    requested byte.
    The read request hasn't finished before returning from do_test().
    After it finishes, it writes the return value and error code from the
    read syscall to the struct aiocb a, which lays on the stack of do_test.
    The stack of the subsequent function call of _dl_fini or _dl_sort_fini,
    which is inlined in _dl_fini is corrupted.
    
    In case of S390, it reads a zero and decrements it by 1:
    unsigned int k = nmaps - 1;
    struct link_map **runp = maps[k]->l_initfini;
    The load from unmapped memory leads to the segmentation fault.
    The stack corruption also happens on other architectures.
    I saw them e.g. on x86 and ppc, too.
    
    This patch adds an aio_suspend call to ensure, that the read request
    is finished before returning from do_test().

    ChangeLog:

    	* nptl/tst-cancel17.c (do_test): Wait for finishing aio_read(&a).

diff --git a/nptl/tst-cancel17.c b/nptl/tst-cancel17.c
index fb89292..9ff4e27 100644
--- a/nptl/tst-cancel17.c
+++ b/nptl/tst-cancel17.c
@@ -333,6 +333,22 @@ do_test (void)
 
   puts ("early cancellation succeeded");
 
+  if (ap == &a2)
+    {
+      /* The aio_read(&a) was not canceled, because the read request was
+	 already in progress. In the meanwhile aio_write(ap) wrote something
+	 to the pipe and the read request either has already been finished or
+	 is able to read the requested byte.
+	 Wait for the read request before returning from this function, because
+	 the return value and error code from the read syscall will be written
+	 to the struct aiocb a, which lays on the stack of this function.
+	 Otherwise the stack from subsequent function calls - e.g. _dl_fini -
+	 will be corrupted, which can lead to undefined behaviour like a
+	 segmentation fault.  */
+      const struct aiocb *l[1] = { &a };
+      TEMP_FAILURE_RETRY (aio_suspend(l, 1, NULL));
+    }
+
   return 0;
 }