linux-kernel - Re: [PATCH v3] mm/madvise: unrestrict process

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Message-ID: <20240927-holzlatten-karierte-2f1e5c0af19d@brauner>
Date: Fri, 27 Sep 2024 10:04:25 +0200
From: Christian Brauner <brauner@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, 
	"Liam R . Howlett" <Liam.Howlett@...cle.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>, pedro.falcato@...il.com
Subject: Re: [PATCH v3] mm/madvise: unrestrict process_madvise() for current
 process

On Thu, Sep 26, 2024 at 08:52:32AM GMT, Shakeel Butt wrote:
> On Thu, Sep 26, 2024 at 04:10:19PM GMT, Lorenzo Stoakes wrote:
> > The process_madvise() call was introduced in commit ecb8ac8b1f14
> > ("mm/madvise: introduce process_madvise() syscall: an external memory
> > hinting API") as a means of performing madvise() operations on another
> > process.
> > 
> > However, as it provides the means by which to perform multiple madvise()
> > operations in a batch via an iovec, it is useful to utilise the same
> > interface for performing operations on the current process rather than a
> > remote one.
> > 
> > Commit 22af8caff7d1 ("mm/madvise: process_madvise() drop capability check
> > if same mm") removed the need for a caller invoking process_madvise() on
> > its own pidfd to possess the CAP_SYS_NICE capability, however this leaves
> > the restrictions on operation in place.
> > 
> > Resolve this by only applying the restriction on operations when accessing
> > a remote process.
> > 
> > Moving forward we plan to implement a simpler means of specifying this
> > condition other than needing to establish a self pidfd, perhaps in the form
> > of a sentinel pidfd.
> > 
> > Also take the opportunity to refactor the system call implementation
> > abstracting the vectorised operation.
> > 
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> 
> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> 
> > ---
> > v3:
> > * Avoid introducing PR_MADV_SELF and defer a non-pidfd version until later.
> 
> Seems like a good plan to decouple this patch from PR_MADV_SELF vs
> PIDFD_SELF decision. I am hoping to see the follow up patch as well.

PIDFD_SELF should absolutely not be a per-system call thing. It should
be generic across all pidfd based system calls similar to AT_FDCWD.

IOW, that should be in:

include/uapi/linux/pidfd.h

#define PIDFD_SELF -200