[PATCH] [v2] 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,

Replace the assignment with an in-place memset() plus assignment that
should always be more efficient for both stack usage and runtime cost.

Fixes: 49889374ab92 ("statmount: simplify string option retrieval")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/namespace.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index d036196f949c..159f1df379fc 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4957,15 +4957,12 @@ static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 	if (!access_ok(buf, bufsize))
 		return -EFAULT;
 
-	*ks = (struct kstatmount){
-		.mask		= kreq->param,
-		.buf		= buf,
-		.bufsize	= bufsize,
-		.seq = {
-			.size	= seq_size,
-			.buf	= kvmalloc(seq_size, GFP_KERNEL_ACCOUNT),
-		},
-	};
+	memset(ks, 0, sizeof(*ks));
+	ks->mask = kreq->param;
+	ks->buf = buf;
+	ks->bufsize = bufsize;
+	ks->seq.size = seq_size;
+	ks->seq.buf = kvmalloc(seq_size, GFP_KERNEL_ACCOUNT);
 	if (!ks->seq.buf)
 		return -ENOMEM;
 	return 0;
-- 
2.39.2