[PATCH v2 03/22] coredump: set the minimum send buffer size

The send buffer of the coredump socket is subject to the limit in
net.core.wmem_default. af_unix uses sk_sndbuf / 2 - 64 bytes for a
single skb. That means a send buffer below that would split a page-sized
write into multiple skbs. Raise the send buffer to leave room for a
page-sized write plus the header. The default value is well above that.
So we really change it when it's below our minimum.

Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
 fs/coredump.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/coredump.c b/fs/coredump.c
index ac4e922c9cb9..24a6405ea242 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -665,6 +665,9 @@ static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
 }
 
 #ifdef CONFIG_UNIX
+/* af_unix halves the send buffer to size a single skb. */
+#define COREDUMP_SOCK_SNDBUF_MIN (3 * PAGE_SIZE)
+
 static bool coredump_sock_connect(struct core_name *cn, struct coredump_params *cprm)
 {
 	struct file *file __free(fput) = NULL;
@@ -690,6 +693,10 @@ static bool coredump_sock_connect(struct core_name *cn, struct coredump_params *
 	if (retval < 0)
 		return false;
 
+	/* Don't let a page-sized write split into several skbs. */
+	socket->sk->sk_sndbuf = max_t(int, socket->sk->sk_sndbuf,
+				      COREDUMP_SOCK_SNDBUF_MIN);
+
 	file = sock_alloc_file(socket, 0, NULL);
 	if (IS_ERR(file))
 		return false;

-- 
2.53.0