Sourceware Bugzilla – Full Text Bug Listing
|Summary:||memcpy acts randomly (and differently) with overlapping areas|
|Product:||glibc||Reporter:||Linus Torvalds <torvalds>|
|Component:||libc||Assignee:||Ulrich Drepper <drepper.fsp>|
|Severity:||normal||CC:||alexander.hunt2005, bvasselle, felipe.contreras, hjl.tools, jakub, jvillalo, kenyon, ml-bz-dale, oliver.henshaw, shaforostoff, smconvey, thierry.vignaud, tmoody, tmraz, tolzmann, tudorb, vincent-srcware, y.pronenko|
Example patch to give the basic idea
Patch to check for overlaps
Patch to check for overlaps
Description Linus Torvalds 2011-02-25 02:26:20 UTC
Created attachment 5264 [details] Example patch to give the basic idea I realize that it's technically "undefined", but the behavior has changed, and in the process broken existing binaries. It's a common bug to use memcpy() instead of memmove(), and the traditional behavior of "copy upwards" means that that bug can go unnoticed for a *long* time if the memory move moves things downwards. The change was introduced in commit 6fb8cbcb58a29fff73eb2101b34caa19a7f88eba ("Improve 64bit memcpy/memmove for Atom, Core 2 and Core i7"), which was part of the 2.13 release. As a result, upgrading from Fedora-13 to Fedora-14 caused applications like flash to fail. But it's a bug that has been reported for other applications too (and it's a bug that I've personally introduced in 'git' too - happily, people had run things like valgrind, so I don't know of any _current_ cases of it). And there is absolutely _zero_ reason to not handle overlapping areas correctly. The "undefined behavior" is due to historical implementation issues, not because it's better than just doing the right thing. And now applications will randomly do different things depending on the phase of the moon (technically, depending on which CPU they have and what particular version of memcpy() glibc happens to choose). So from a pure Quality-of-Implementation standpoint, just make glibc implement memcpy() as memmove(), if you don't want to re-instate the traditional behavior that binaries have at least been tested with. Don't break random existing binaries just because the glibc version changed, and they had never been tested with the new random behavior! I'm attaching an EXAMPLE patch against the current glibc git tree: it just tries to get rid of the unnecessary differences between memcpy and memmove for the normal ssse3 case. The approach is simple: - small copies (less than 80 bytes) have hand-coded optimized code that gets called through a jump table. Those cases already handle overlapping areas correctly (simply because they do all loads up-front, and stores at the end), and they are used for memmove() as-is. So change the memcpy() function to test for the small case first, since that's the one that can be latency critical. - once we're talking about bigger memcpy() cases, the extra couple of cycles to check "which direction should I copy" are totally immaterial, and having two different versions of basically the same code is just silly and is quite likely to cost more than (in I$ and page fault overhead) than just doing it in the same code. So just remove all the USE_AS_MEMMOVE games. I haven't actually tested the patch, since building glibc is black magic and the simple "just alias __memmove_ssse3 to __memcpy_sse3" thing didn't work for me and resulted in linker errors. There's probably some simple (but subtle) magic reason for that, that I simply don't know about. So take the patch as a "hey, doing it this way should be simpler and avoid the problem" rather than "apply this patch as-is". The ssse3-back case (which prefers backwards copies) needs similar treatment, I'll happily do that and test it if people just let me know that it's worth my time (and tell me what the black magic incantation is).
Comment 1 Linus Torvalds 2011-02-25 02:29:37 UTC
Btw, this has hit quite a lot of people. It's Fedora https://bugzilla.redhat.com/show_bug.cgi?id=638477 and there are examples of having other things than just flash break (like old gstreamer plugins etc)
Comment 2 H.J. Lu 2011-02-25 03:57:42 UTC
Although the current memcpy in glibc follows the spec, some applications call memcpy with overlapping destination and source. I think we should help those applications without punishing the correctly written applications. We can check an environment variable for IFUNC, like LD_BIND_IFUNC_MEMCPY_TO_MEMMOVE. If it is set, IFUNC memcpy will return memmove instead of memcpy. I can prepare a patch to implement it if we should do it.
Comment 3 Linus Torvalds 2011-02-25 04:14:39 UTC
So is there any real reason to believe that memmove() can't just be as fast as memcpy? Seriously. That's what it all boils down to. Why have a separate memcpy() at all, when it clearly is correct - and nice to people - to always just implement it as a memmove(). And I really don't see the downside. It's not like it's going to be slower. People who want to check whether their application is portable can use valgrind, the same way you have to check for portability issues in other areas (eg the extra glibc specific printf formats etc - they just "work", but they certainly aren't portable). So why not just be nice. If anything, from a "be nice" standpoint, it would perhaps be good to have a "debug mode", that would actually _warn_ - or even assert - about overlapping memcpy(). That obviously shouldn't be the default (since that only helps developers), but it would be along the same lines of the nice malloc debugging help that glibc can give. IOW, I think this is an area where glibc can be _better_ than what is required of it.
Comment 4 Ulrich Drepper 2011-02-25 09:59:18 UTC
The existence of code written by people who should never have been allowed to touch a keyboard cannot be allowed to prevent a correct implementation. If there is a large bod of code out there we can work around the issue for that. We can have the existing memcpy@GLIBC_2.2.5 implementation work around the issue by avoiding the backward copying code. A new memcpy@GLIBC_2.14 could use the code currently in use. Whether the current, new memcpy is only slightly faster than one mimicking memmove is really not that important. There are going to be more and different implementations in the future and they shouldn't be prevented from being used because of buggy programs. We should be happy to have this code here and now. With this proposed implementation old code remains in working order until those lousy programmers use a newer platform and then they will experience the problems themselves and will fix them before releasing their code. I'm happy to entertain a patch to this effect.
Comment 5 Jakub Jelinek 2011-02-25 12:07:14 UTC
If we go that route (symbol versioning memcpy), then wouldn't it be better to just alias the old memcpy@GLIBC_2.2.5 to memmove and have the new memcpy@@GLIBC_2.14 be the only memcpy implementation? Having two sets of memcpy implementations (especially when we have many memcpy implementations on x86_64 and i?86 selectable via STT_GNU_IFUNC), even if the workaroundish one is placed in compat .text subsections, would waste too much code section in libc.so.6.
Comment 6 H.J. Lu 2011-02-25 15:19:47 UTC
(In reply to comment #5) > If we go that route (symbol versioning memcpy), then wouldn't it be better to > just alias the old memcpy@GLIBC_2.2.5 to memmove and have the new > memcpy@@GLIBC_2.14 be the only memcpy implementation? Having two sets of memcpy I like this idea.
Comment 7 Ulrich Drepper 2011-02-26 01:56:18 UTC
(In reply to comment #5) > If we go that route (symbol versioning memcpy), then wouldn't it be better to > just alias the old memcpy@GLIBC_2.2.5 to memmove and have the new > memcpy@@GLIBC_2.14 be the only memcpy implementation? That's what I have in mind.
Comment 8 Felipe Contreras 2011-03-03 23:27:33 UTC
"Buggy" code can be linked to the new memcpy@@GLIBC_2.14 by just recompiling, no? That doesn't really solve anything. Sure, code should use memcpy correctly, and if glibc has a compelling reason to break these programs, it should. As Ulrich mentions in comment #4 "There are going to be more and different implementations in the future and they shouldn't be prevented from being used because of buggy programs." But _today_ that's not the case. Today, the regression can be fixed easily with a patch like what Linus is proposing, and there will be no *downside* whatsoever. How about glibc breaks the behavior _only_ when there's an *upside* to breaking it?
Comment 9 Ulrich Drepper 2011-03-04 23:41:41 UTC
(In reply to comment #8) > "Buggy" code can be linked to the new memcpy@@GLIBC_2.14 by just recompiling, > no? That doesn't really solve anything. It solves everything. If you just relink without retesting you're an idiot and acting irresponsible.
Comment 10 Linus Torvalds 2011-03-04 23:53:36 UTC
(In reply to comment #9) > > It solves everything. If you just relink without retesting you're an idiot and > acting irresponsible. It does solve a lot, and at least fixes the "you broke stuff that used to work" issue. However, I still don't understand why you guys can't just admit that you might as well just do memmove() and be done with it. It's not slower. And the excuse that "you'll have more implementations in the future" is just an even stronger reason to do that. To make this very clear: your new "and improved" memcpy() ACTS DIFFERENTLY ON DIFFERENT MACHINES. That means that all that testing that was done by the developer at link-time is ALMOST TOTALLY WORTHLESS, because what was tested wasn't necessarily at all what the user sees. I really don't understand why you can't admit that random behavior like that by a very fundamental core library routine is actually a real problem. And there really isn't any upside. The optimized routines up to 80 bytes are the same (in fact, my patch should speed them up by a few cycles), and anything bigger than that will not notice the extra couple of cycles to check for overlap. So while I agree that it's a fix to the immediate problem to just say "don't screw up for existing binaries", I do NOT understand why the glibc people then apparently think it's fine to be stupid for new binaries.
Comment 11 Felipe Contreras 2011-03-05 00:28:38 UTC
(In reply to comment #9) > (In reply to comment #8) > > "Buggy" code can be linked to the new memcpy@@GLIBC_2.14 by just recompiling, > > no? That doesn't really solve anything. > > It solves everything. If you just relink without retesting you're an idiot and > acting irresponsible. You cannot test every possible code-path.
Comment 12 Oliver Henshaw 2011-03-05 13:28:28 UTC
The thread starting at http://lwn.net/Articles/414496/ may be instructive on how this can cause problems despite the good(-ish) intentions of the developer. In short, squashfs used memcpy with data that was known not to overlap but later changes invalidated this assumption.
Comment 13 Bruno Vasselle 2011-03-24 21:03:57 UTC
(In reply to comment #9) > (In reply to comment #8) > > "Buggy" code can be linked to the new memcpy@@GLIBC_2.14 by just recompiling, > > no? That doesn't really solve anything. > > It solves everything. If you just relink without retesting you're an idiot and > acting irresponsible. Nobody is interested in an optimization in lib C that would at best gain less than the simple fact of calling the function. Everybody is interested in using the code that idiots may produce.
Comment 14 H.J. Lu 2011-03-25 04:37:07 UTC
Created attachment 5323 [details] A patch This works for me.
Comment 15 Jakub Jelinek 2011-03-25 06:45:50 UTC
This is not correct: 1) glibc 2.13 has been released already, so new symbol versions must be GLIBC_2.14 2) you do it only on x86_64, therefore you should add it into sysdeps/x86_64/Versions (though, you will need to add GLIBC_2.14 to toplevel Versions.def too)
Comment 16 Felipe Contreras 2011-03-28 11:53:48 UTC
So we have two solutions: 1) The solution proposed by Linus Torvalds, attachment #5264 [details] Advantages: everything works the same as before Disadvantages: a few extra cycles in certain memcpy's 2) The solution proposed by Ulrich Drepper, attachment #5323 [details] Advantages: old binaries keep working Disadvantages: newly compiled code remains affected Clearly 1) is the most sensible solution as the only advantage of 2) is a few cycles, and has drastic disadvantages.
Comment 17 Felipe Contreras 2011-03-29 14:43:31 UTC
Actually, why not have both? I think this plan would fit everyone: 1) Apply Linus' patch Both to 2.13 and 2.14, this would not introduce any issues and would restore back the previous behavior, so applications would still work. As I mentioned before, the disadvantages are minimal. 2) Apply H.J. Lu's patch This would go to 2.14, but not only to x86 but all architectures, and add an overlap check, and when triggered issue a crash. This would allow old binaries to keep working on 2.14, but newly compiled ones would crash if they are doing something wrong. This would allow all the users of glibc to fix their code for the impending changes. 3) Remove overlap checks On 2.15, after a transition period, the overlap checks can be removed, and thus save the few extra cycles. This has all the advantages of all the proposals, and makes it easy to fix the applications that are using memcpy wrong.
Comment 18 Bruno Vasselle 2011-03-29 22:27:52 UTC
(In reply to comment #17) > Actually, why not have both? I think this plan would fit everyone: No, it does not. It certainly does not. It is not only the problem of recompiling the existing code, it's also the problem of fixing it and re-qualifying it. This plan has a huge cost... and it's vain. The contract C programmers have with C and the C library is clear: we accept a fair amount of inefficency, but we don't have to program in assembly nor care about the system's internals. How many people still use the C library when it comes to be important to gain an addition plus a comparison? The problem we're facing just made this fact plain: there is no reason why memcpy should not be memmove.
Comment 19 Felipe Contreras 2011-03-30 05:14:25 UTC
(In reply to comment #18) > (In reply to comment #17) > > Actually, why not have both? I think this plan would fit everyone: > > No, it does not. It certainly does not. > > It is not only the problem of recompiling the existing code, it's also the > problem of fixing it and re-qualifying it. This plan has a huge cost... and > it's vain. I understand that, but it's better than the current alternative that Ulrich is more likely to merge, which is basically to do nothing.
Comment 20 Felipe Contreras 2011-03-30 07:21:26 UTC
Created attachment 5341 [details] Patch to check for overlaps I'm trying this patch to find out how many things in the system use overlapping memcpy, however, it doesn't seem to cause any crashes... Can anyone spot something wrong?
Comment 21 shaforostoff 2011-03-31 08:36:56 UTC
i'd like to vote for having memcpy and memmove identical. (unless there are benchmarks that are showing that perf difference is high)
Comment 22 Ulrich Drepper 2011-04-01 23:41:31 UTC
HJ's patch which implements what I proposed is in git.
Comment 23 H.J. Lu 2011-04-02 02:57:19 UTC
(In reply to comment #3) > > If anything, from a "be nice" standpoint, it would perhaps be good to have a > "debug mode", that would actually _warn_ - or even assert - about overlapping > memcpy(). That obviously shouldn't be the default (since that only helps You can use LD_AUDIT to do it today.
Comment 24 Bruno Vasselle 2011-04-10 15:39:30 UTC
(In reply to comment #4) > The existence of code written by people who should never have been allowed to > touch a keyboard cannot be allowed to prevent a correct implementation. If > there is a large bod of code out there we can work around the issue for that. > Don't you see it is just NOT a correct implementation?
Comment 25 Felipe Contreras 2011-04-10 15:49:59 UTC
Created attachment 5660 [details] Patch to check for overlaps Actually the code from Linus had a bug in the 'cmp' checks, here's the updated version. I just started to run my Fedora system with this, and I already see crashes in pulseaudio and readahead-collector. I will continue running this for a bit, but I think it's pretty clear that we should not assume that all applications in a typical system have been using memcpy properly. So, again, I think we need at least a transition period so that people can detect and fix the issues.
Comment 26 Vincent Lefèvre 2011-05-04 13:29:06 UTC
(In reply to comment #18) > The problem we're facing just made this fact plain: there is no reason why > memcpy should not be memmove. If these two functions should behave in the same way, then why not all programmers use memmove (which has fewer requirements)? If memcpy is called while the objects overlap, the bug is not necessarily that memmove should have been used instead. The cause may be an incorrect size. So, with this point of view, it should be safer to abort than letting the program behave in an uncontrolled way. (In reply to comment #25) > So, again, I think we need at least a transition period so that people can > detect and fix the issues. But it may be difficult to detect the issues. For instance, zsh was affected by a similar problem (now fixed in CVS only) with the optimized strcpy, but to detect the problem, it involves keyboard input: http://www.zsh.org/mla/workers/2011/msg00533.html http://www.zsh.org/mla/workers/2011/msg00544.html For strcpy, this is even worse, as there is no strmove function, so that either programmers have to write non-portable code or they have to reimplement a naive version of strcpy or find some other workaround, such as memmove + strlen: http://www.zsh.org/mla/workers/2011/msg00542.html I suppose that if this has been done for memcpy, then strcpy should also be patched in some way...
Comment 27 Felipe Contreras 2011-05-04 15:01:05 UTC
(In reply to comment #26) > (In reply to comment #25) > > So, again, I think we need at least a transition period so that people can > > detect and fix the issues. > > But it may be difficult to detect the issues. For instance, zsh was affected by > a similar problem (now fixed in CVS only) with the optimized strcpy, but to > detect the problem, it involves keyboard input: Yes, it is difficult, that's why I think it should be enabled globally. BTW. Here's another issue on zsh I found while enabling the memcpy checks: https://bugzilla.redhat.com/show_bug.cgi?id=698439
Comment 28 Tudor Bosman 2013-08-12 17:19:54 UTC
FYI, this bug has bitten me in a different way: memcpy() copying backwards defeats the MADV_SEQUENTIAL flag to madvise(). A trivial file copier implementation (mmap source, mmap destination, set MADV_SEQUENTIAL, memcpy from source to destination) would perform much worse on machines that support SSSE3 than on machines that don't because of this bug. (Before anyone tells me that I should copy files using read() and write(), my actual usage pattern was more complex, but the details are irrelevant.)
Comment 29 Ondrej Bilka 2013-08-19 23:48:50 UTC
On Mon, Aug 12, 2013 at 05:19:54PM +0000, tudorb at gmail dot com wrote: > --- Comment #28 from Tudor Bosman <tudorb at gmail dot com> --- > FYI, this bug has bitten me in a different way: memcpy() copying backwards > defeats the MADV_SEQUENTIAL flag to madvise(). A trivial file copier > implementation (mmap source, mmap destination, set MADV_SEQUENTIAL, memcpy from > source to destination) would perform much worse on machines that support SSSE3 > than on machines that don't because of this bug. > Wait, do you have overlapping source and destination areas? If so then a backward copy is necessary. Or is backward copy called for not overlapping areas? I have a patch that could call backward copy only if src-n < dest < src+n holds. > (Before anyone tells me that I should copy files using read() and write(), my > actual usage pattern was more complex, but the details are irrelevant.) > > -- > You are receiving this mail because: > You are on the CC list for the bug.
Comment 30 Tudor Bosman 2013-08-20 01:12:11 UTC
(In reply to Ondrej Bilka from comment #29) > Wait, do you have overlapping source and destination areas? If so then a > backward copy is necessary. Backward copy is often used for non-overlapping areas as it is faster on some Intel processors; see https://bugzilla.redhat.com/show_bug.cgi?id=638477#c99 for an explanation. I was pointing out that it also has the (unintended) behavior of pessimizing mmap()ed file accesses.