selftests/clone3: fix wild pointer access of getline due to missing init

selftests/clone3: fix wild pointer access of getline due to missing init

Patch series "selftests: Add missing initalization of pointer passed to
getline", v2.

This patch (of 2):

Clone3_set_tid uses getline(&line, ...) in a loop to read the child's
process status.  The code expects that getline allocates the buffer for
the line on the first loop iteration.  According to the Open Group
Spec[1], char *line has to be null pointer for this:

> ssize_t getline(char **restrict lineptr, ...);
> If *lineptr is a null pointer or if the object pointed to by *lineptr
> is of insufficient size, an object shall be allocated as if by
malloc()
> or the object shall be reallocated as if by realloc()[...].

However, char *line is only declared, leading to an undefined value that
is potentially non-null.  In an example run with Musl v1.2.6, the realloc
call[2] of getdelim, which implements getline, triggers a segfault:

./run_kselftest.sh --test clone3:clone3_set_tid
[ 1366.165898] kselftest: Running tests in clone3
...
[ 1367.799244] clone3_set_tid[811]: unhandled signal 11 code 0x1 at
0x0000000000000000 in libc.so[68184,3fbf69f000+4c000]
[ 1367.802808] CPU: 0 UID: 0 PID: 811 Comm: clone3_set_tid Not tainted
..
[ 1367.804188]  epc: 0x0000003fbf6b0184
[ 1367.804188]  ra : 0x0000003fbf6d4664
[ 1367.804188]  sp : 0x0000003fce5f2e40
[ 1367.805314]  gp : 0x0000002aaab0dfb8
[ 1367.805314]  tp : 0x0000003fbf6f14a8
[ 1367.805314]  t0 : 0x0000003fbf63d000
...

Looking at the realloc implementation, Musl mallocs for a null pointer
memory.  But for a non-null pointer, it assumes it's passed a valid
pointer to the heap and tries to access its meta-data.  This leads to the
segfault we see:

void *realloc(void *p, size_t n)
{
        if (!p) return malloc(n);
        if (size_overflows(n)) return 0;

        struct meta *g = get_meta(p);
        ...
}

Fix this by properly initializing the line pointer to NULL.

Link: https://lore.kernel.org/20260722130246.2135563-1-christian.gellermann@codasip.com
Link: https://lore.kernel.org/20260722130246.2135563-2-christian.gellermann@codasip.com
Link: https://pubs.opengroup.org/onlinepubs/9799919799/functions/getline.html
Link: https://git.musl-libc.org/cgit/musl/tree/src/stdio/getdelim.c#n38
Fixes: 41585bbeeef9 ("selftests: add tests for clone3() with *set_tid")
Signed-off-by: Chris Gellermann <christian.gellermann@codasip.com>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>