linux-kernel - [PATCH] sendfile: Do not update file offset of non-lseek()able objects.

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Date:   Mon, 12 Jun 2017 19:14:29 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: [PATCH] sendfile: Do not update file offset of non-lseek()able objects.

I tried to sendfile() a file which is larger than 4GB to a pipe (which is
the stdout of Apache's CGI program), and noticed that sendfile() fails with
EFBIG after 2GB is copied to stdout pipe. This is because sendfile() is
updating file offset of the file descriptor of the pipe.

sendfile() should not update file offset if the file descriptor refers to
an non-lseek()able object.

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
 fs/read_write.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 47c1d44..17ea13c 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1459,7 +1459,8 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
 		add_wchar(current, retval);
 		fsnotify_access(in.file);
 		fsnotify_modify(out.file);
-		out.file->f_pos = out_pos;
+		if (out.file->f_op->llseek != no_llseek)
+			out.file->f_pos = out_pos;
 		if (ppos)
 			*ppos = pos;
 		else
-- 
1.8.3.1