Messages in this thread | | | Date | Tue, 26 Nov 2013 13:54:54 -0800 | From | Andrew Morton <> | Subject | Re: copy_from_user_*() and buffer zeroing |
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On Tue, 26 Nov 2013 13:07:07 -0800 "H. Peter Anvin" <hpa@zytor.com> wrote:
> I just started looking into the horribly confused state of buffer > zeroing for the various copy_from_user variants. This came up after we > did some minor tuning last week. > > copy_from_user_inatomic() seems to be documented to not zero the buffer. > This is definitely *NOT* true on x86-64, although it does seem to be > true on i386 -- on x86-64, we carry along a "zerorest" flag but in all > possible codepaths it will be set to true unless the remaining byte > count is zero anyway. > > Furthermore, on at least x86-64, if we do an early bailout, we don't > zero the entire buffer in the case of a hard-coded 10- or 16-byte buffer > (why only those sizes is anybody's guess.) See lines 71-88 of uaccess_64.h. > > I'd like to figure out what is the required and what is the desirable > behavior here, and then fix the code accordingly. >
Nine years ago:
commit 7079f897164cb14f616c785d3d01629fd6a97719 Author: mingo <mingo> Date: Fri Aug 27 17:33:18 2004 +0000
[PATCH] Add a few might_sleep() checks Add a whole bunch more might_sleep() checks. We also enable might_sleep() checking in copy_*_user(). This was non-trivial because of the "copy_*_user() in atomic regions" trick would generate false positives. Fix that up by adding a new __copy_*_user_inatomic(), which avoids the might_sleep() check. Only i386 is supported in this patch.
I can't think of any reason why __copy_from_user_inatomic() should be non-zeroing. But maybe I'm missing something - this would pretty easily permit uninitialised data to appear in pagecache and someone surely would have noticed..
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