fs/select: rework stack allocation hack for clang

A while ago, we changed the way that select() and poll() preallocate
a temporary buffer just under the size of the static warning limit of
1024 bytes, as clang was frequently going slightly above that limit.

The warnings have recently returned and I took another look. As it turns
out, clang is not actually inherently worse at reserving stack space,
it just happens to inline do_select() into core_sys_select(), while gcc
never inlines it.

Annotate do_select() to never be inlined and in turn remove the special
case for the allocation size. This should give the same behavior for
both clang and gcc all the time and once more avoids those warnings.

Fixes: ad312f95d4 ("fs/select: avoid clang stack usage warning")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240216202352.2492798-1-arnd@kernel.org
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/select.c

@@ -476,7 +476,7 @@ static inline void wait_key_set(poll_table *wait, unsigned long in,
 		wait->_key |= POLLOUT_SET;
 }
 
-static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
+static noinline_for_stack int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
 {
 	ktime_t expire, *to = NULL;
 	struct poll_wqueues table;

include/linux/poll.h

@@ -14,11 +14,7 @@
 
 /* ~832 bytes of stack space used max in sys_select/sys_poll before allocating
    additional memory. */
-#ifdef __clang__
-#define MAX_STACK_ALLOC 768
-#else
 #define MAX_STACK_ALLOC 832
-#endif
 #define FRONTEND_STACK_ALLOC	256
 #define SELECT_STACK_ALLOC	FRONTEND_STACK_ALLOC
 #define POLL_STACK_ALLOC	FRONTEND_STACK_ALLOC