Re: [PATCHv3 perf/core 07/22] uprobes: Add do_ref_ctr argument to uprobe_write function

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 25 Jun 2025 17:11:09 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Wed, Jun 25, 2025 at 03:42:59PM +0900, Masami Hiramatsu wrote:
> > On Thu,  5 Jun 2025 15:23:34 +0200
> > Jiri Olsa <jolsa@kernel.org> wrote:
> > 
> > > Making update_ref_ctr call in uprobe_write conditional based
> > > on do_ref_ctr argument. This way we can use uprobe_write for
> > > instruction update without doing ref_ctr_offset update.
> > > 
> > 
> > Can we just decouple this update from uprobe_write()?
> > If we do this exclusively, I think we can do something like;
> > 
> > lock()
> > update_ref_ctr(uprobe, mm, +1);
> > ...
> > ret = uprobe_write();
> > ...
> > if (ret < 0)
> >   update_ref_ctr(uprobe, mm, -1);
> > unlock()
> > 
> > Thank you,
> 
> it was the intention in the v1 but as Oleg pointed out [1] it won't
> work, because the set_orig_refctr part of this change does not know
> if the refctr should be really updated or not

Ah, uprobe_unregister() can not check the given uprobe failed to
update the code for each mm....

> 
> while inside uprobe_write it happens right after verify callback and
> we make sure to update refctr only if the instruction was updated

OK. But this is very complicated. Eventually I think uprobe should
have a list of mm(or refcnt) to manage whether it is updated the
refcounter.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,

> 
> thanks,
> jirka
> 
> 
> [1] https://lore.kernel.org/bpf/20250427141335.GA9350@redhat.com/
> 
> > 
> > 
> > > Acked-by: Oleg Nesterov <oleg@redhat.com>
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > >  include/linux/uprobes.h | 2 +-
> > >  kernel/events/uprobes.c | 8 ++++----
> > >  2 files changed, 5 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> > > index 518b26756469..5080619560d4 100644
> > > --- a/include/linux/uprobes.h
> > > +++ b/include/linux/uprobes.h
> > > @@ -200,7 +200,7 @@ extern unsigned long uprobe_get_trap_addr(struct pt_regs *regs);
> > >  extern int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma, unsigned long vaddr, uprobe_opcode_t,
> > >  			       bool is_register);
> > >  extern int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma, const unsigned long opcode_vaddr,
> > > -			uprobe_opcode_t *insn, int nbytes, uprobe_write_verify_t verify, bool is_register);
> > > +			uprobe_opcode_t *insn, int nbytes, uprobe_write_verify_t verify, bool is_register, bool do_update_ref_ctr);
> > >  extern struct uprobe *uprobe_register(struct inode *inode, loff_t offset, loff_t ref_ctr_offset, struct uprobe_consumer *uc);
> > >  extern int uprobe_apply(struct uprobe *uprobe, struct uprobe_consumer *uc, bool);
> > >  extern void uprobe_unregister_nosync(struct uprobe *uprobe, struct uprobe_consumer *uc);
> > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > > index 1e5dc3b30707..6795b8d82b9c 100644
> > > --- a/kernel/events/uprobes.c
> > > +++ b/kernel/events/uprobes.c
> > > @@ -492,12 +492,12 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> > >  		bool is_register)
> > >  {
> > >  	return uprobe_write(auprobe, vma, opcode_vaddr, &opcode, UPROBE_SWBP_INSN_SIZE,
> > > -			    verify_opcode, is_register);
> > > +			    verify_opcode, is_register, true /* do_update_ref_ctr */);
> > >  }
> > >  
> > >  int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> > >  		 const unsigned long insn_vaddr, uprobe_opcode_t *insn, int nbytes,
> > > -		 uprobe_write_verify_t verify, bool is_register)
> > > +		 uprobe_write_verify_t verify, bool is_register, bool do_update_ref_ctr)
> > >  {
> > >  	const unsigned long vaddr = insn_vaddr & PAGE_MASK;
> > >  	struct mm_struct *mm = vma->vm_mm;
> > > @@ -538,7 +538,7 @@ int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> > >  	}
> > >  
> > >  	/* We are going to replace instruction, update ref_ctr. */
> > > -	if (!ref_ctr_updated && uprobe->ref_ctr_offset) {
> > > +	if (do_update_ref_ctr && !ref_ctr_updated && uprobe->ref_ctr_offset) {
> > >  		ret = update_ref_ctr(uprobe, mm, is_register ? 1 : -1);
> > >  		if (ret) {
> > >  			folio_put(folio);
> > > @@ -590,7 +590,7 @@ int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> > >  
> > >  out:
> > >  	/* Revert back reference counter if instruction update failed. */
> > > -	if (ret < 0 && ref_ctr_updated)
> > > +	if (do_update_ref_ctr && ret < 0 && ref_ctr_updated)
> > >  		update_ref_ctr(uprobe, mm, is_register ? -1 : 1);
> > >  
> > >  	/* try collapse pmd for compound page */
> > > -- 
> > > 2.49.0
> > > 
> > 
> > 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>