Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 1 Jul 2020 13:51:11 -0700 | Subject | Re: objtool clac/stac handling change.. |
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On Wed, Jul 1, 2020 at 1:36 PM Andy Lutomirski <luto@amacapital.net> wrote: > > We ought to be able to do it the way I described and get decent code generation too.
No, we really can't.
Each access really needs to jump to an exception label. Otherwise any time you have multiple operations (think "strncpy()" and friends) you have to test in between each access.
That is why *fundamnetally* the interface to "unsafe_get/put_user()" takes a label for the error case. There is absolutely no way to make any other interface work efficiently.
(Unless, of course, you make the exception handling something that the compiler does entirely on its own. But that has never been a good idea for the kernel, and I wouldn't trust a compiler to do what the kernel needs).
Side note: the labels can be hidden. I did (long ago) send out something that did a
uaccess_try { val1 = unsafe_get_user(addr); val2 = unsafe_get_user(addr2); } uaccess_catch { error handling here };
kind of thing, but that was just syntactic wrapper around that label model. And honestly, it doesn't really change anything fundamental, it really ends up with exactly the same issues just with a slightly different syntax.
(I did that because we had the nasty "put_user_ex()" interfaces, which were horrible horrible crap, and if one access took an exception, then all the other ones did too).
The "label for error case" is actually simpler to follow both for the user and for a compiler. Yes, it's a bit odd, but once you get used to it, it's really quite regular. But having a different error handler for the "user_access_begin()" failure and the actual access failure really does end up generating duplicate code and confusion.
Linus
Linus
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