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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 8AA041C0006 X-Stat-Signature: c1qrjqzw9we9dbzci5sq9gertadg3o8s X-Rspam-User: X-HE-Tag: 1749068909-532578 X-HE-Meta: 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 5PnySazS a2s7/mx0eTGS1bUIN4XCkOrHCM0r+zeWuTS1rHOexITpIFX/43zQgP4rGcOfmJxSc0+F4nWFpeD0DF0u6bjBMta2eg5/9W6DwcNHplpximd8HfZi6PTp0yqRQvQ2vsygBG8U3ayrA6AcFHmBIIvWummN2Y3BgMEFBFJ6FdNjpS+ZW4JMj2SZisAf4FIGGctZuUeIBbjGYjwQ9BkbMzArGzXeU1yZcinNS11peHG848alEhfUJplBdTi04dKmjRB8kKfDF/pjqPTm0f65Oe6hWHjlz01rwhw+K0kL/1Dw3rkowfgB9DyNPnZdLT33l7fX2zp3ElM994nV5PRMYMAKnFKFZ3izUs3wA4fET7E/joDoX3qioQAsE/g8jAbRnDH7HXvuV+rA0fbx0JXvO8emFOmwFlNtNTHoY9UpSvQ5fyUQb9chVDtnJAlY0AmmpZioNDcxC2u8oZ1+skqLYOBwFhO/QlCak6TvBN54W X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 04.06.25 22:10, Peter Xu wrote: > On Wed, Jun 04, 2025 at 08:11:08PM +0200, Jann Horn wrote: >> On Wed, Jun 4, 2025 at 7:04 PM Peter Xu wrote: >>> On Tue, Jun 03, 2025 at 08:21:03PM +0200, Jann Horn wrote: >>>> It is not currently documented that the child of fork() should receive a >>>> coherent snapshot of the parent's memory, or how we get such a snapshot. >>>> Add a comment block to explain this. >>>> >>>> Signed-off-by: Jann Horn >>>> --- >>>> kernel/fork.c | 34 ++++++++++++++++++++++++++++++++++ >>>> 1 file changed, 34 insertions(+) >>>> >>>> diff --git a/kernel/fork.c b/kernel/fork.c >>>> index 85afccfdf3b1..f78f5df596a9 100644 >>>> --- a/kernel/fork.c >>>> +++ b/kernel/fork.c >>>> @@ -604,6 +604,40 @@ static void dup_mm_exe_file(struct mm_struct *mm, struct mm_struct *oldmm) >>>> } >>>> >>>> #ifdef CONFIG_MMU >>>> +/* >>>> + * Anonymous memory inherited by the child MM must, on success, contain a >>>> + * coherent snapshot of corresponding anonymous memory in the parent MM. >>> >>> Should we better define what is a coherent snapshot? Or maybe avoid using >>> this term which seems to apply to the whole mm? >>> >>> I think it's at least not a snapshot of whole mm at a specific time, >>> because as long as there can be more than one concurrent writers (hence, it >>> needs to be at least 3 threads in the parent process, 1 in charge of fork), >>> this can happen: >>> >>> parent writer 1 parent writer 2 parent fork thr >>> --------------- --------------- --------------- >>> wr-protect P1 >>> write P1 <---- T1 >>> (trapped, didn't happen) >>> write PN <---- T2 >>> (went through) >>> ... >>> wr-protect PN >>> >>> The result of above would be that child process will see a mixture of old >>> P1 (at timestamp T1) but updated P2 (timestamp T2). I don't think it's >>> impossible that the userapp could try to serialize "write P1" and "write >>> PN" operations in a way that it would also get a surprise seeing in the >>> child PN updated but P1 didn't. >> >> If the write at T1 hits a page fault, then it doesn't actually happen >> at T1. The write instruction starts doing something at T1, but it does >> not fully retire, and the architectural register state does not >> change, and in particular the instruction pointer does not advance >> past this instruction; just like when speculative execution is aborted >> after a branch misprediction, except that the CPU raises an exception >> and we enter the page fault handler. The write actually happens when >> the instruction is executed a second time after page fault handling >> has completed after the mmap lock is dropped. (Unless something during >> page fault handling raises a signal, in which case the instruction >> might never architecturally execute.) > > Fair enough. So maybe that's something like a best-effort whole mm > snapshot anytime happened during the fork() but before releasing mmap write > lock. > > Your comment did mention one exception on the kernel, is it still pretty > easy to happen? I'm thinking this use case of trying to load some data > from a O_DIRECT fd and then set the var to show it's loaded: > > bool data_read=0 > read(...); > data_read=1; > > Then IIUC this can happen: > > parent thread 1 parent fork thr > --------------- --------------- > read(...) > using O_DIRECT on priv-anon buffers P1 > pin_user_pages > fork() happens > Sees P1 pinned > P1 early COW (child sees no data loaded) > memcpy() > set data_read=1 > (data_read can be a global private var on P2) > P2 wr-protected (child sees data_read=1) > > Hence in child even if it sees data_read=1 it is possible the buffer may be > uninitialized, or the buffer is partly loaded, still racing with the kernel > early COW. Just mentioning that O_DIRECT and fork() has had a problematic relationship for a long time, although we are getting better at handling it (IOW, not break common setups in nasty ways). "man open" is still quite verbose on that "O_DIRECT I/Os should never be run concurrently with the fork(2) system call ..." -- Cheers, David / dhildenb