Messages in this thread | | | Date | Thu, 22 Apr 2010 17:40:59 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2 7/11] Uprobes Implementation |
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On 04/22, Srikar Dronamraju wrote: > > * Oleg Nesterov <oleg@redhat.com> [2010-04-21 18:05:15]: > > > 3. mprotect(). write_opcode() checks !VM_WRITE. This is correct, > > otherwise we can race with the user-space writing to the same > > page. > > > > 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? > > > > I still need to verify this. I shall get back to you on this. > However are there applications that mprotect(PROT_WRITE) text pages?
Well, I think the kernel should assume that the user-space can do anything.
Hmm. And if this vma is VM_SHARED, then this bp could be actually written to vm_file after mprotect().
But I think this doesn't really matter. When I actually look at patches 3 and 4, I am starting to think this all is very wrong.
> I am copying Mel Gorman and Andrea Arcangeli so that they can provide > their inputs on VM and KSM related issues.
Yes. We need vm experts here, I am not. Still, I'd like to share my concerns. I also added Rik and Hugh.
So, 3/11 does
@@ -2617,7 +2617,10 @@ int replace_page(struct vm_area_struct *vma, struct page *page, } get_page(kpage); - page_add_anon_rmap(kpage, vma, addr); + if (PageAnon(kpage)) + page_add_anon_rmap(kpage, vma, addr); + else + page_add_file_rmap(kpage); flush_cache_page(vma, addr, pte_pfn(*ptep)); ptep_clear_flush(vma, addr, ptep);
I see no point in this patch, please see below.
The next 4/11 patch introduces write_opcode() which roughly does:
int write_opcode(unsigned long vaddr, user_bkpt_opcode_t opcode) { get_user_pages(write => false, &old_page);
new_page = alloc_page_vma(...);
... insert the bp into the new_page ...
new_page->mapping = old_page->mapping; new_page->index = old_page->index;
replace_page(old_page, new_page); }
This doesn't look right at all to me.
IF PageAnon(old_page):
in this case replace_page() calls page_add_anon_rmap() which needs the locked page.
ELSE:
I don't think the new page should evere preserve the mapping, this looks just wrong. It should be always anonymous.
And in fact, I do not understand why write_opcode() needs replace_page(). It could just use get_user_pages(FOLL_WRITE | FOLL_FORCE), no? It should create the anonymous page correctly.
Either way, I think register_uprobe() should disallow the probes in VM_SHARED/VM_MAYWRITE vmas.
Oleg.
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