Messages in this thread | | | Date | Tue, 18 Nov 2014 13:39:26 -0800 | Subject | Re: [RFC] situation with csum_and_copy_... API | From | Linus Torvalds <> |
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On Tue, Nov 18, 2014 at 1:23 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > > OK... If netdev folks can live with that for now, I've no problem with > dropping 3/5. However, I really think we need a variant of csum-and-copy > that would _not_ bother with access_ok() longer term. That can wait, though...
iirc,access_ok() ends up being something like three or four instructions. It's generally not very painful.
The main reason to ever use the "__" functions is for __get_user() or __put_user() in a loop (or when doing multiple ones when loading/storing a struct or a signal stack frame or whatever), and then the real issue ends up being that the __get_user() is really often just a single instruction, while "get_user()" is a function call that does that access_ok().
So then moving the access_ok() outside the loop, or to above the structure load, can make a *big* deal. But removing the access_ok() entirely tends to not be a huge issue - it's just that you want to do it *once*, instead of doing it over-and-over.
There might be some case you really want to remove it from the function entirely so that the "access_ok()" is no longer close to the accesses it checks, but I really think you want to have a goof performance case for it, and a comment about it.
And no, we haven't really always followed those rules.
Btw, these days, on x86, we actually have a bigger issue with the whole STAC/CLAC thing: it's a good safety measure, but doing STAC/CLAC for each access is painful. So "__get_user()" and "__put_user()" sadly aren't the single-instruction things they used to be any more. The code sequences that really want tight code (think the 'struct stat' copying and the like) sadly cannot get it as it is..
Linus
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