Messages in this thread | | | Date | Fri, 9 Mar 2012 13:46:37 -0800 | From | Matt Helsley <> | Subject | Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3 |
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On Thu, Mar 08, 2012 at 11:03:03PM +0400, Cyrill Gorcunov wrote: > On Thu, Mar 08, 2012 at 07:26:23PM +0100, Oleg Nesterov wrote: > > On 03/08, Cyrill Gorcunov wrote: > > > > > > Hi Oleg, could you please take a look once you get a minute (no urgency). > > > > Add Matt. I won't touch the text below to keep the patch intact. > > Thanks for CC'ing Matt, Oleg (I forgot, sorry). > > > > > With this change > > > > down_write(&mm->mmap_sem); > > if (mm->num_exe_file_vmas) { > > fput(mm->exe_file); > > mm->exe_file = exe_file; > > exe_file = NULL; > > } else > > set_mm_exe_file(mm, exe_file); > > up_write(&mm->mmap_sem); > > > > I simply do not understand what mm->num_exe_file_vmas means after > > PR_SET_MM_EXE_FILE.
I think it should fail if the num_exe_file_vmas is not 0 when PR_SET_MM_EXE_FILE is used. It's simple, keeps things clear, might catch userspace bugs (harder to accidentally leave a mapping of the original executable), and could avoid kernel bugs too.
> > > > I think that you should do > > > > down_write(&mm->mmap_sem); > > if (mm->num_exe_file_vmas) { > > fput(mm->exe_file); > > mm->exe_file = exe_file; > > exe_file = NULL; > > } > > up_write(&mm->mmap_sem); > > > > to keep the current "mm->exe_file goes away after the final > > unmap(MAP_EXECUTABLE)" logic. > > > > OK, may be this doesn't work in c/r case because you are actually > > going to remove the old mappings? But in this case the new exe_file > > will go away anyway, afaics PR_SET_MM_EXE_FILE is called when you > > still have the old mappings. > > Yes, exactly, I need to remove old mappings first (because VMAs > we're about to restore may intersect with current map the host > program has). And yes, once they all are removed I don't have > /proc/pid/exe anymore. That's why I need num_exe_file_vmas == 0 > case. > > When I setup new exe_file with num_exe_file_vmas = 0, this reference > to a file brings /proc/pid/exe back to live (and when process exiting > it'll call set_mm_exe_file(mm, NULL) and the new exe_file will be dropped, > so no leak here).
Makes sense, I think.
> > And I don't think the unconditional > > > > down_write(&mm->mmap_sem); > > set_mm_exe_file(mm, exe_file); > > up_write(&mm->mmap_sem); > > > > is 100% right, this clears ->num_exe_file_vmas. This means that > > (if you still have the old mapping) the new exe_file can go away > > after added_exe_file_vma() + removed_exe_file_vma(). Normally this > > should happen, but afaics this is possible. Note that even, say, > > mprotect() can trigger added_exe_file_vma(). > > > > Wait, Oleg, I'm confused, in case if there *is* exitsting VM_EXECUTABLEs > then we jump into first banch and simply replace old exe_file.
What happens if multiple prctl calls are made? We'll have a mix of N executable files that've been mapped n_i times. I think we're better off just returning an error in that case -- -EBUSY or something.
> If there is no VM_EXECUTABLEs, then we simply setup new exe_file > and num_exe_file_vmas remains zero.
Which is fine.
Cheers, -Matt
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