linux-kernel - Re: [PATCH v5 tip/perf/core 1/2] uprobes: simplify find_active_uprobe

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Message-ID: <CAEf4BzZf5C-tqbKqJxPaoPZi0tLky8Y8wCS8k-+MekNxvoHTog@mail.gmail.com>
Date: Tue, 26 Nov 2024 20:49:04 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jann Horn <jannh@...gle.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-trace-kernel@...r.kernel.org, 
	linux-mm@...ck.org, akpm@...ux-foundation.org, peterz@...radead.org, 
	mingo@...nel.org, torvalds@...ux-foundation.org, oleg@...hat.com, 
	rostedt@...dmis.org, mhiramat@...nel.org, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, jolsa@...nel.org, paulmck@...nel.org, 
	willy@...radead.org, surenb@...gle.com, mjguzik@...il.com, brauner@...nel.org, 
	mhocko@...nel.org, vbabka@...e.cz, shakeel.butt@...ux.dev, hannes@...xchg.org, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, david@...hat.com, 
	arnd@...db.de, viro@...iv.linux.org.uk, hca@...ux.ibm.com
Subject: Re: [PATCH v5 tip/perf/core 1/2] uprobes: simplify
 find_active_uprobe_rcu() VMA checks

On Tue, Nov 26, 2024 at 2:20 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Fri, Nov 22, 2024 at 4:59 AM Andrii Nakryiko <andrii@...nel.org> wrote:
> > At the point where find_active_uprobe_rcu() is used we know that VMA in
> > question has triggered software breakpoint, so we don't need to validate
> > vma->vm_flags. Keep only vma->vm_file NULL check.
>
> How do we know that the VMA we find triggered a software breakpoint?
> Between the time a software breakpoint was hit and the time we took
> the mmap_read_lock(), the VMA could have been replaced with an
> entirely different one, right?

We need that VMA only to get inode + file offset, and whether it is
the original VMA with uprobe installed, or someone raced and replaced
it with some other VMA shouldn't matter. We either find uprobe at that
offset within that inode, or not. So this seems fine.

>
> I don't know this code well, and your change looks like it's probably
> fine (since the file is just used to look up its inode in some tree,
> and therefore for incompatible files, the lookup is guaranteed to fail
> and nothing will happen). But I think the commit message looks dodgy.
>
> > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > Acked-by: Oleg Nesterov <oleg@...hat.com>
> > Suggested-by: Oleg Nesterov <oleg@...hat.com>
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> > ---
> >  kernel/events/uprobes.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index a76ddc5fc982..c4da8f741f3a 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -2305,7 +2305,7 @@ static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swb
> >         mmap_read_lock(mm);
> >         vma = vma_lookup(mm, bp_vaddr);
> >         if (vma) {
> > -               if (valid_vma(vma, false)) {
> > +               if (vma->vm_file) {
> >                         struct inode *inode = file_inode(vma->vm_file);
> >                         loff_t offset = vaddr_to_offset(vma, bp_vaddr);
> >
> > --
> > 2.43.5
> >