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sysfs: fix off-by-one error in fill_read_buffer

PATCH In review Miao Xie 2 participants 4 messages Nov 8, 2007 – Nov 13, 2007 Driver core

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Miao Xie

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#0 lore raw

PATCH 1/1 sysfs: fix off-by-one error in fill_read_buffer

Hi,everyone.
I found that there is a off-by-one problem in the following code.

Version: 2.6.24-rc2
File: fs/sysfs/file.c:118-122
Function: fill_read_buffer
--------------------------------------------------------------------

	count = ops->show(kobj, attr_sd->s_attr.attr, buffer->page);

	sysfs_put_active_two(attr_sd);

	BUG_ON(count > (ssize_t)PAGE_SIZE);

--------------------------------------------------------------------
Because according to the specification of the sysfs and the implement of the show methods, the show methods return the number of bytes which would be generated for the given input, excluding the trailing null.So if the return value of the show methods equals PAGE_SIZE - 1, the buffer is full in fact. And if the return value equals PAGE_SIZE, the resulting string was already truncated,or buffer overflow occurred.

This patch fixes an off-by-one error in fill_read_buffer.

Signed-off-by Miao Xie <[email protected]>
---
 fs/sysfs/file.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
fs/sysfs/file.c +1 -1
index 27d1785..f98d66d 100644
@@ -119,7 +119,7 @@ static int fill_read_buffer(struct dentry * dentry, struct sysfs_buffer * buffer
 
 	sysfs_put_active_two(attr_sd);
 
-	BUG_ON(count > (ssize_t)PAGE_SIZE);
+	BUG_ON(count >= (ssize_t)PAGE_SIZE);
 	if (count >= 0) {
 		buffer->needs_read_fill = 0;
 		buffer->count = count;
Signature · 1 line
1.5.3
#1 lore raw
Quoted 21 lines · Miao Xie

Hi,everyone.
I found that there is a off-by-one problem in the following code.

Version:    2.6.24-rc2
File:        fs/sysfs/file.c:118-122
Function:    fill_read_buffer
--------------------------------------------------------------------
    count = ops->show(kobj, attr_sd->s_attr.attr, buffer->page);

    sysfs_put_active_two(attr_sd);

    BUG_ON(count > (ssize_t)PAGE_SIZE);

--------------------------------------------------------------------
Because according to the specification of the sysfs and the implement of the
show methods, the show methods return the number of bytes which would be generated for the given input, excluding the trailing null.So if the return value of the show methods equals PAGE_SIZE - 1, the buffer is full in fact. And if the return value equals PAGE_SIZE, the resulting string was already truncated,or buffer overflow occurred.

Quoted 3 lines

This patch fixes an off-by-one error in fill_read_buffer.

Signed-off-by Miao Xie <[email protected]>

It isn't strictly a bug. If the ->show() op fills full PAGE_SIZE and returns PAGE_SIZE, the user will get full PAGE_SIZE bytes correctly, so it will work. However, considering normal use cases, return value of PAGE_SIZE very likely indicates an error condition, so considering it a BUG condition is a good idea.

Miao, can you please note that the code works fine with PAGE_SIZE return but it's likely to indicate truncated result or overflow in normal use cases as a comment on top of the BUG_ON()?

Thanks.

Signature · 1 line
tejun
author
#2 lore raw
Quoted 11 lines · on 2007-11-12 22:08 Tejun Heo wrote:

It isn't strictly a bug. If the ->show() op fills full PAGE_SIZE and returns PAGE_SIZE, the user will get full PAGE_SIZE bytes correctly, so it will work. However, considering normal use cases, return value of PAGE_SIZE very likely indicates an error condition, so considering it a BUG condition is a good idea.

Miao, can you please note that the code works fine with PAGE_SIZE return but it's likely to indicate truncated result or overflow in normal use cases as a comment on top of the BUG_ON()?

Thanks.

OK, I did it.

Signed-off-by Miao Xie <[email protected]>
---
 fs/sysfs/file.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
fs/sysfs/file.c +3 -1
index 27d1785..7f6a8d2 100644
@@ -119,7 +119,9 @@ static int fill_read_buffer(struct dentry * dentry, struct sysfs_buffer * buffer
 
 	sysfs_put_active_two(attr_sd);
 
-	BUG_ON(count > (ssize_t)PAGE_SIZE);
+	/* the code works fine with PAGE_SIZE return but it's likely to
+	   indicate truncated result or overflow in normal use cases. */
+	BUG_ON(count >= (ssize_t)PAGE_SIZE);
 	if (count >= 0) {
 		buffer->needs_read_fill = 0;
 		buffer->count = count;
Signature · 1 line
1.5.3
Acked-by
#3 lore raw
Quoted 11 lines · Miao Xie
Quoted 10 lines · on 2007-11-12 22:08 Tejun Heo wrote:

It isn't strictly a bug. If the ->show() op fills full PAGE_SIZE and returns PAGE_SIZE, the user will get full PAGE_SIZE bytes correctly, so it will work. However, considering normal use cases, return value of PAGE_SIZE very likely indicates an error condition, so considering it a BUG condition is a good idea.

Miao, can you please note that the code works fine with PAGE_SIZE return but it's likely to indicate truncated result or overflow in normal use cases as a comment on top of the BUG_ON()?

Quoted 5 lines
Quoted 1 line

Thanks.

OK, I did it.

Signed-off-by Miao Xie <[email protected]>
Acked-by Tejun Heo <[email protected]>

Greg, can you please push this one? Thanks.

Signature · 1 line
tejun