Messages in this thread | | | Date | Wed, 12 May 2010 15:55:18 +0530 | From | Srikar Dronamraju <> | Subject | Re: [PATCH v3 0/10] Uprobes v3 |
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* Peter Zijlstra <peterz@infradead.org> [2010-05-11 22:59:45]:
> On Thu, 2010-05-06 at 23:31 +0530, Srikar Dronamraju wrote: > > - Addressed comments from Oleg, including removal of interrupt context > > handlers, reverting background page replacement in favour of > > access_process_vm(). > > > > +static int write_opcode(struct task_struct *tsk, unsigned long vaddr, > > + user_bkpt_opcode_t opcode) > > +{ > > + int ret; > > + > > + if (!tsk) > > + return -EINVAL; > > + > > + ret = access_process_vm(tsk, vaddr, &opcode, user_bkpt_opcode_sz, 1); > > + return (ret == user_bkpt_opcode_sz ? 0 : -EFAULT); > > +} > > Why! > > That's not not the atomic sequence outlined. >
Yes, we had moved away from access_process_vm to background page replacement in Version 1 and Version 2.
One of the reasons being Mathieu suggesting to Jim in LFCS that for almost all architectures insertion of a breakpoint instruction on a user page is an atomic operation, as far as the CPU is concerned.
Can you and other VM experts tell me if access_process_vm isnt going to be atomic with respect to inserting/deleting a breakpoint instruction?
Oleg had few questions which I didnt have answers. (Most of which you have already answered yesterday). One thing that's still missing is
[ snipping from Oleg's mail: ] ----- But suppose that the application does mprotect(PROT_WRITE) after register_uprobe() installs the bp, now unregister_uprobe/etc can't restore the original insn? ---
Also I tried a write_opcode that uses background page replacement which addressed some of Oleg's comments. The pseudo-code is here: write_opcode() { down_read(mmap_sem);
get_user_pages(tsk, mm, vaddr, .. &old_page, &vma);
anon_vma_prepare(vma);
new_page=alloc_page_vma(.., vma, vaddr);
copy_user_page(new_page, old_page);
kmap_atomic(new_page,...);
memcpy(vaddr,..);
kunmap_atomic(..);
lock_page(new_page);
old_pte = get_pte(mm,vaddr);
replace_page(vma, new_page, old_page, old_pte);
unlock_page(new_page);
put_page(new_page);
put_page(old_page);
up_read(mmap); }
Will this work?
The Other VM quieries that I had were:
Is there any thing else needed for the parent process to pass on the anon_vma to the child process. (I inserted a breakpoint in the parent and tried removing the breakpoint in the child. However page_address_in_vma() (called by replace_page() returned EFAULT because "vma->anon_vma != page_anon_vma(page)"
Do we need to take care of mem_cgroups? Do we need to update mm counters?
-- Thanks and Regards Srikar
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