Messages in this thread | | | From | Linus Torvalds <> | Date | Sun, 29 Mar 2020 10:56:59 -0700 | Subject | Re: [RFC][PATCH 01/22] x86 user stack frame reads: switch to explicit __get_user() |
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On Sun, Mar 29, 2020 at 10:41 AM David Laight <David.Laight@aculab.com> wrote: > > It may be worth implementing get_user() as an inline > function that writes the result of access_ok() to a > 'by reference' parameter and then returns the value > from an 'real' __get_user() function.
That's how get_user() already works.
It is a polymorphic function (done using macros, sizeof() and ugly compiler tricks) that generates a call, yes. But it's not a normal C call. On x86-64, it returns the error code in %rax, and the value in %rdx
So "get_user()" is already basically optimal. It's likely *faster* than __get_user(), because it has a smaller I$ footprint if you do multiple ones.
But, if you have lots of performance-critical get_user() calls, just use
if (user_access_begin(..)) goto efault;
.. multiple "unsafe_get_user(x,ptr,efault);" ..
user_access_end(); ...
efault: user_access_end(); return -EFAULT;
and be done with it.
Yes, the above sequence looks cumbersome, but it's designed for doing multiple accesses together efficiently. It's basically the "I actually had a good reason to use __get_user(), but it sucks now, so this is the new interface"
It's designed for multiple accesses, because as mentioned, if you only have one, then "get_user()" is already optimal.
And yes, the interface (with that "label for error cases") is optimized for a (future) world where the compiler can do "asm goto" together with outputs. Any exception on the access doesn't actually generate a test at all, the exception will branch directly to the error label instead.
That already works for "unsafe_put_user()", but for "unsafe_get_user()" you need a compiler that can do that kind of "asm goto".
If you use a modern clang version (ie build clang from git), I can send you a patch for the kernel to try (and a patch for clang to fix a bug, unless it's been already merged, I didn't check).
The above will generate basically _optimal_ code with my patch and that modern clang version.
Linus
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