Re: Accessing file-offset info for fds in /proc?

From: Dave Kleikamp
Date: Tue Feb 20 2007 - 08:44:02 EST

On Tue, 2007-02-20 at 02:31 -0500, Hank Leininger wrote:
> Is there anything provided by the kernel that would let you see the
> current offset of an existing filehandle?
> Sometimes when processing a very large file (grepping a log, bzip2'ing
> or gpg'ing a file, or whatever), I'd really like to know how far along
> it is, because I'm impatient. lsof has an -o flag to show offsets for
> file descriptors it lists, but it appears that's not supported under
> Linux. It looks like all of the information lsof and fuser print about
> files in use, etc can be gotten from /proc/*/fd/* (and /proc/*/maps, but
> I'm not really concerned with mmap'ed files, just positions on fds).
> Sometimes I'll resort to strace -s4096'ing the process to see what chunk
> of text it's currently reading, and try to guess from that. Silly.
> Has anybody ever developed a patch to implement this? I realize this
> could create a variety of information-leakage problems; the information
> probably would need to be restricted, such as by the same rules as
> dumpable. Are there any horribly painful reasons why this couldn't be
> done?

It shouldn't be too painful. The code to populate /proc/*/fd/ has the
file struct. It just doesn't have a place pass the offset to user-space
since it's basically creating a symlink. In proc_fd_link(), it has the
file struct. The offset is file->f_pos.

One could create something like /proc/*/fd_offsets, whose read method
could list the file descriptor, path, and offset for each open file.

David Kleikamp
IBM Linux Technology Center

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