[PATCH v2] proc: Use seq_read_iter for /proc/*/maps

["Arkadiusz Kozdra (Arusekk)" <arek_koz@o2.pl>]

Since seq_read_iter looks mature enough to be used for /proc/<pid>/maps,
re-allow applications to perform zero-copy data forwarding from it.

Some executable-inspecting tools rely on patching entry point
instructions with minimal machine code that uses sendfile to read
/proc/self/maps to stdout.  The sendfile call allows them to do it
faster and without excessive allocations.

This is inspired by the series by Cristoph Hellwig (linked).

Changes since v1:

only change proc_pid_maps_operations

Link: https://lore.kernel.org/lkml/20201104082738.1054792-1-hch@lst.de/
Fixes: 36e2c7421f02 ("fs: don't allow splice read/write without explicit ops")
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arkadiusz Kozdra (Arusekk) <arek_koz@o2.pl>
---
 fs/proc/task_mmu.c   | 3 ++-
 fs/proc/task_nommu.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index e862cab69583..06282294ddb8 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -351,7 +351,8 @@ static int pid_maps_open(struct inode *inode, struct file *file)
 
 const struct file_operations proc_pid_maps_operations = {
 	.open		= pid_maps_open,
-	.read		= seq_read,
+	.read_iter	= seq_read_iter,
+	.splice_read    = generic_file_splice_read,
 	.llseek		= seq_lseek,
 	.release	= proc_map_release,
 };
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index a6d21fc0033c..e55e79fd0175 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -295,7 +295,8 @@ static int pid_maps_open(struct inode *inode, struct file *file)
 
 const struct file_operations proc_pid_maps_operations = {
 	.open		= pid_maps_open,
-	.read		= seq_read,
+	.read_iter	= seq_read_iter,
+	.splice_read	= generic_file_splice_read,
 	.llseek		= seq_lseek,
 	.release	= map_release,
 };
-- 
2.31.1