Re: [PATCH] process_vm_{read,write}v(3): initial man pages

On Tuesday 20 March 2012 02:27:31 Michael Kerrisk (man-pages) wrote:

On Tue, Mar 20, 2012 at 1:50 PM, Mike Frysinger [off-list ref] wrote:

quoted

On Monday 19 March 2012 16:45:41 Michael Kerrisk (man-pages) wrote:

quoted

A quick question about one piece:

quoted

+The count values might be individually capped according to
\fIUIO_MAXIOV\fP. +If the Linux kernel is capped at smaller values,
the C library will take care +of emulating the limit it exposes (if
it is bigger) so the user only needs to +care about that (what the C
library defines).

I don't see anything in glibc that does this. Have I missed something?

i think you're correct.  the code in glibc atm is merely a syscall().  i
think the idea was to have the C library guarantee that and if moving
forward the kernel changes, the C library would update by adding a
wrapper.  maybe just drop this sentence until that day comes ?

So, does the kernel currently impose a limit on the size of the iovec?
It wasn't immediately clear to me from a quick scan of the source.

yes.  process_vm_{read,write}v() in mm/process_vm_access.c calls 
process_vm_rw() which calls rw_copy_check_uvector() in fs/read_write.c and one 
of the first things it does:
    if (nr_segs > UIO_MAXIOV) {
        ret = -EINVAL;
        goto out;
    }
-mike