Re: [PATCHv2 perf/core 01/22] uprobes: Remove breakpoint in unapply_uprobe under mmap_write_lock

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, May 20, 2025 at 08:48:45AM +0900, Masami Hiramatsu wrote:
> On Thu, 15 May 2025 14:10:58 +0200
> Jiri Olsa <jolsa@kernel.org> wrote:
> 
> > Currently unapply_uprobe takes mmap_read_lock, but it might call
> > remove_breakpoint which eventually changes user pages.
> > 
> > Current code writes either breakpoint or original instruction, so
> > it can probably go away with that, but with the upcoming change that
> > writes multiple instructions on the probed address we need to ensure
> > that any update to mm's pages is exclusive.
> > 
> 
> So, this is a bugfix, right?

nope, the current code is fine (I think), but the new code needs to go
through 2 separate instructions changes and we determine the state of
optimization based on the instruction we find, so we need to be sure
there's only one thread inside remove_breakpoint call

jirka

> 
> Thanks,
> 
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/events/uprobes.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 84ee7b590861..257581432cd8 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -483,7 +483,7 @@ static int __uprobe_write_opcode(struct vm_area_struct *vma,
> >   * @opcode_vaddr: the virtual address to store the opcode.
> >   * @opcode: opcode to be written at @opcode_vaddr.
> >   *
> > - * Called with mm->mmap_lock held for read or write.
> > + * Called with mm->mmap_lock held for write.
> >   * Return 0 (success) or a negative errno.
> >   */
> >  int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> > @@ -1464,7 +1464,7 @@ static int unapply_uprobe(struct uprobe *uprobe, struct mm_struct *mm)
> >  	struct vm_area_struct *vma;
> >  	int err = 0;
> >  
> > -	mmap_read_lock(mm);
> > +	mmap_write_lock(mm);
> >  	for_each_vma(vmi, vma) {
> >  		unsigned long vaddr;
> >  		loff_t offset;
> > @@ -1481,7 +1481,7 @@ static int unapply_uprobe(struct uprobe *uprobe, struct mm_struct *mm)
> >  		vaddr = offset_to_vaddr(vma, uprobe->offset);
> >  		err |= remove_breakpoint(uprobe, vma, vaddr);
> >  	}
> > -	mmap_read_unlock(mm);
> > +	mmap_write_unlock(mm);
> >  
> >  	return err;
> >  }
> > -- 
> > 2.49.0
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@kernel.org>