Messages in this thread | | | Date | Tue, 26 Nov 2013 13:07:07 -0800 | From | "H. Peter Anvin" <> | Subject | copy_from_user_*() and buffer zeroing |
| |
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.
-hpa
| |