linux-kernel - Re: [GIT] /proc/uptime fix

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Date:	Mon, 3 Nov 2008 10:14:16 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alexey Dobriyan <adobriyan@...il.com>,
	Al Viro <viro@...IV.linux.org.uk>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT] /proc/uptime fix



On Mon, 3 Nov 2008, Alexey Dobriyan wrote:
> 
> Alexey Dobriyan (1):
>       proc: revert /proc/uptime to ->read_proc hook

Ok, pulled, but it's a bit sad. 

Al: the commit is 6c87df37dcb9c6c33923707fa5191e0a65874d60, and the 
message explains it all:

    Turned out some VMware userspace does pread(2) on /proc/uptime, but
    seqfiles currently don't allow pread() resulting in -ESPIPE.
    
    Seqfiles in theory can do pread(), but this can be a long story,
    so revert to ->read_proc until then.
    
and I bet that the pread() usage is just with a constant loff_t of 0, and 
just there to avoid a simple case of "lseek+read". 

I wonder how hard it is to make seqfiles support pread. Maybe it's 
something as trivial as just flushing the buffer whenever 'pos' doesn't 
match f_pos?

ie something like the following (TOTALLY UNTESTED!)

I may be way off base here, Al needs to whack some sense into this.

		Linus

---
 fs/seq_file.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index eba2eab..eb95ec9 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -48,8 +48,8 @@ int seq_open(struct file *file, const struct seq_operations *op)
 	 */
 	file->f_version = 0;
 
-	/* SEQ files support lseek, but not pread/pwrite */
-	file->f_mode &= ~(FMODE_PREAD | FMODE_PWRITE);
+	/* SEQ files support lseek, but not pwrite */
+	file->f_mode &= ~(FMODE_PWRITE);
 	return 0;
 }
 EXPORT_SYMBOL(seq_open);
@@ -91,6 +91,8 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 		if (!m->buf)
 			goto Enomem;
 	}
+	if (pos != file->f_pos)
+		m->count = 0;
 	/* if not empty - flush it first */
 	if (m->count) {
 		n = min(m->count, size);
@@ -175,6 +177,8 @@ Done:
 	else
 		*ppos += copied;
 	file->f_version = m->version;
+	if (pos != file->f_pos)
+		m->count = 0;
 	mutex_unlock(&m->lock);
 	return copied;
 Enomem:
--
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