[PATCH] statmount: reduce runtime stack usage

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

prepare_kstatmount() constructs a copy of 'struct kstatmount' on the stack
and copies it into the local variable on the stack of its caller. Because
of the size of this structure, this ends up overflowing the limit for
a single function's stack frame when prepare_kstatmount() gets inlined
and both copies are on the same frame without the compiler being able
to collapse them into one:

fs/namespace.c:4995:1: error: stack frame size (1536) exceeds limit (1024) in '__se_sys_statmount' [-Werror,-Wframe-larger-than]
 4995 | SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,

Mark the inner function as noinline_for_stack so the second copy is
freed before calling do_statmount() enters filesystem specific code.
The extra copy of the structure is a bit inefficient, but this
system call should not be performance critical.

Fixes: 49889374ab92 ("statmount: simplify string option retrieval")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/namespace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index d036196f949c..e22fb5c4a9bb 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4950,7 +4950,8 @@ static inline bool retry_statmount(const long ret, size_t *seq_size)
 	return true;
 }
 
-static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
+static int noinline_for_stack
+prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 			      struct statmount __user *buf, size_t bufsize,
 			      size_t seq_size)
 {
-- 
2.39.2