Re: proc_file_read() hack?

Todd Inglett (
Wed, 27 Mar 2002 12:26:39 -0600

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Thomas Hood wrote:

> Unfortunately, your method #3 conflicts with methods #0 through #2,
> which exhaust the range of possible values that may be returned
> in *start. Any value greater than buffer is regarded as being
> "within the buffer".

I guess I don't understand the conflict.

There is case #0 where start is NULL. In this case start is computed in
proc_file_read as page + *ppos so (unsigned long)start < PROC_BLOCK_SIZE
is NOT true and so start is used as before.

In case #1 (unsigned long)start < PROC_BLOCK_SIZE so it will grab the
data from page and use start as the length to copy as it did before.

And finally cases #2 and #3 are the same: use start as an explicit data
address. It doesn't matter whether this points into page or if it is
space provided by read_proc (which is why I suggested not even
mentioning a "case #3").

Did you get a chance to read the patch? I'll attach it again just in
case. Or is there a chance that start >= PROC_BLOCK_SIZE (but start <
page) in case #1? If that is true I am wondering how it could possibly
be correct since start will be used as a length which is greater than
the size of the page.


> Introducing method #1 was a bad idea because this hack made it
> impossible cleanly to implement what you suggest.
> --
> Thomas Hood
> On Mon, 2002-03-25 at 13:18, Todd Inglett wrote:
>>How about applying my trivial patch and then adding this to your nice
>>3) Set *start = an address outside the buffer.
>> Put the data of the requested offset at *start.
>> Return the number of bytes of data placed there.
>> If this number is greater than zero and you
>> didn't signal eof and the reader is prepared to
>> take more data you will be called again with the
>> requested offset advanced by the number ob tyes
>> absorbed.
>>The code should still work with the other cases now that the hack is
>>fixed. Of course, rather than add 3), it would be better to re-word 2)
>>(e.g. "Set *start = address of the buffer which may or may not be in the
>>given buffer.).
>>There are cases where the data is available and need not be copied. My
>>code got simpler when I got rid of the need to copy my data around.

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Index: fs/proc/generic.c
RCS file: /cvs/linuxppc64/linuxppc64_2_4/fs/proc/generic.c,v
retrieving revision 1.6
diff -u -r1.6 generic.c
--- fs/proc/generic.c 8 Oct 2001 19:19:59 -0000 1.6
+++ fs/proc/generic.c 22 Mar 2002 14:34:47 -0000
@@ -104,14 +104,14 @@
* return the bytes, and set `start' to the desired offset
* as an unsigned int. -
- n -= copy_to_user(buf, start < page ? page : start, n);
+ n -= copy_to_user(buf, (unsigned long)start < PROC_BLOCK_SIZE ? page : start, n);
if (n == 0) {
if (retval == 0)
retval = -EFAULT;

- *ppos += start < page ? (long)start : n; /* Move down the file */
+ *ppos += (unsigned long)start < PROC_BLOCK_SIZE ? (long)start : n; /* Move down the file */
nbytes -= n;
buf += n;
retval += n;


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