Messages in this thread | | | Subject | Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the fast path | From | "H. Peter Anvin" <> | Date | Sun, 17 Apr 2016 23:01:27 -0700 |
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On 04/17/16 22:48, Andy Lutomirski wrote: > > I think I prefer the "reject weird input" behavior over the "accept > and normalize weird input" if we can get away with it, and I'm fairly > confident that we can get away with "reject weird input" given that > distro kernels do exactly that already. >
It's not "weird", it is the ABI as defined. We have to do this for all the system call arguments, too; you just don't notice it because the compiler does it for us. Some other architectures, e.g. s390, has the opposite convention where the caller is responsible for normalizing the result; in that case we have to do it *again* in the kernel, which is one of the major reasons for the SYSCALL_*() macros.
So I'm not sure this is a valid consideration. The reason it generally works is because the natural way for the user space code to work is to load a value into %eax which will naturally zero-extend to %rax, but it isn't inherently required to work that way.
-hpa
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