On Tue, Sep 24, 2024 at 01:51:11PM GMT, Pedro Falcato wrote:

On Tue, Sep 24, 2024 at 12:16:27PM GMT, Lorenzo Stoakes wrote:

quoted

process_madvise() was conceived as a useful means for performing a vector
of madvise() operations on a remote process's address space.

However it's useful to be able to do so on the current process also. It is
currently rather clunky to do this (requiring a pidfd to be opened for the
current process) and introduces unnecessary overhead in incrementing
reference counts for the task and mm.

Avoid all of this by providing a PR_MADV_SELF flag, which causes
process_madvise() to simply ignore the pidfd parameter and instead apply
the operation to the current process.

How about simply defining a pseudo-fd PIDFD_SELF in the negative int space?
There's precedent for it in the fs space (AT_FDCWD). I think it's more ergonomic
and if you take out the errno space we have around 2^31 - 4096 available sentinel
values.

e.g:

/* AT_FDCWD = -10, -1 is dangerous, pick a different value */
#define PIDFD_SELF   -11

int pidfd = target_pid == getpid() ? PIDFD_SELF : pidfd_open(...);
process_madvise(pidfd, ...);


What do you think?

I like the way you're thinking, but I don't think this is something we can
do in the context of this series.

I mean, I totally accept using a flag here and ignoring the pidfd field is
_ugly_, no question. But I'm trying to find the smallest change that
achieves what we want.

To add such a sentinel would be a change to the pidfd mechanism as a whole,
and we'd be left in the awkward situation that no other user of the pidfd
mechanism would be implementing this, but we'd have to expose this as a
general sentinel value for all pidfd users.

One nice thing with doing this as a flag is that, later, if somebody is
willing to do the larger task of having a special sentinel pidfd value to
mean 'the current process', we could use this in process_madvise() and
deprecate this flag :)

--
Pedro