Messages in this thread Patch in this message | | | Date | Wed, 27 Mar 2002 12:26:39 -0600 | From | Todd Inglett <> | Subject | Re: proc_file_read() hack? |
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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.
-todd
> > 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 >>comment? >> >>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. >>
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. - Paul.Russell@rustcorp.com.au */ - 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; break; } - *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; | |