[LTP] [PATCH] syscalls/kcmp03: work around bug in C library

[LTP] [PATCH] syscalls/kcmp03: work around bug in C library

[Steve Muckle]

Some versions of glibc and bionic report the same PID for parent and
child when clone() is called with CLONE_VM but not CLONE_THREAD. This
causes an issue in the cleanup path which is supposed to execute in the
parent. If kcmp() support is not present, the child aborts and then
erroneously runs the cleanup handler, freeing its own stack.

Avoid this by statically allocating the stack.

Signed-off-by: Steve Muckle <smuckle.linux@gmail.com>
---
 testcases/kernel/syscalls/kcmp/kcmp03.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/testcases/kernel/syscalls/kcmp/kcmp03.c b/testcases/kernel/syscalls/kcmp/kcmp03.c
index b6ca40cff..465cb1e8b 100644
--- a/testcases/kernel/syscalls/kcmp/kcmp03.c
+++ b/testcases/kernel/syscalls/kcmp/kcmp03.c
@@ -40,7 +40,13 @@
 
 static int pid1;
 static int pid2;
-static void *stack;
+
+/*
+ * The cleanup handler may be run in the child due to a bug in some versions of
+ * glibc and bionic when clone() is called with CLONE_VM but not CLONE_THREAD.
+ * Use static allocation of stack to avoid this.
+ */
+static char stack[STACK_SIZE];
 
 static struct tcase {
 	int clone_type;
@@ -52,16 +58,6 @@ static struct tcase {
 	{CLONE_SYSVSEM, KCMP_SYSVSEM}
 };
 
-static void setup(void)
-{
-	stack = SAFE_MALLOC(STACK_SIZE);
-}
-
-static void cleanup(void)
-{
-	free(stack);
-}
-
 static int do_child(void *arg)
 {
 	pid2 = getpid();
@@ -96,8 +92,6 @@ static void verify_kcmp(unsigned int n)
 
 static struct tst_test test = {
 	.tcnt = ARRAY_SIZE(tcases),
-	.setup = setup,
-	.cleanup = cleanup,
 	.forks_child = 1,
 	.test = verify_kcmp,
 	.min_kver = "3.5.0"
-- 
2.14.1.581.gf28d330327-goog

[LTP] [PATCH] syscalls/kcmp03: work around bug in C library

[Cyril Hrubis]

Hi!
> Some versions of glibc and bionic report the same PID for parent and
> child when clone() is called with CLONE_VM but not CLONE_THREAD.

Is that the result of the infamous pid caching? Can we fix that by using
the raw getpid() syscall in the test library? If so I guess that using
raw syscall in the test library may be a safer bet.

-- 
Cyril Hrubis
chrubis@suse.cz

[LTP] [PATCH] syscalls/kcmp03: work around bug in C library

[Steve Muckle]

On 09/06/2017 03:09 AM, Cyril Hrubis wrote:
> Hi!
>> Some versions of glibc and bionic report the same PID for parent
>> and child when clone() is called with CLONE_VM but not
>> CLONE_THREAD.
> 
> Is that the result of the infamous pid caching?

In bionic yes, I can't say for sure in glibc but I'd assume so. I
haven't tested a glibc version that had pid caching removed.

> Can we fix that by using the raw getpid() syscall in the test
> library? If so I guess that using raw syscall in the test library may
> be a safer bet.
Probably. I'll try it.

thanks,
Steve