From: Lennert Buytenhek

Sent: 28 January 2021 23:07

On Sun, Jan 24, 2021 at 10:21:38PM +0000, David Laight wrote:

quoted

quoted

One open question is whether IORING_OP_GETDENTS64 should be more like
pread(2) and allow passing in a starting offset to read from the
directory from.  (This would require some more surgery in fs/readdir.c.)

Since directories are seekable this ought to work.
Modulo horrid issues with 32bit file offsets.

The incremental patch below does this.  (It doesn't apply cleanly on
top of v1 of the IORING_OP_GETDENTS patch as I have other changes in
my tree -- I'm including it just to illustrate the changes that would
make this work.)

This change seems to work, and makes IORING_OP_GETDENTS take an
explicitly specified directory offset (instead of using the file's
->f_pos), making it more like pread(2), and I like the change from
a conceptual point of view, but it's a bit ugly around
iterate_dir_use_ctx_pos().  Any thoughts on how to do this more
cleanly (without breaking iterate_dir() semantics)?

I had a further thought...
I presume the basic operation is:
	lock(file);
	do_getents(); // Updates file->offset
	unlock(file);

Which means you can implement an offset by saving, updating
and restoring file->offset while the lock is held.

This is a bit like the completely broken pread() in uclibc
which uses two lseek() calls to set and restore the offset.
Whoever wrote that needs shooting - worse than useless.

Glibc is as bad:
	// Don't even ask what glibc's clock_nanosleep() does, you don't want to know.
	while (syscall(SYS_clock_nanosleep, CLOCK_MONOTONIC, TIMER_ABSTIME, &ts, NULL)

   David

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