Messages in this thread |  | | From | Linus Torvalds <> | Date | Sun, 4 Feb 2018 10:40:36 -0800 | Subject | Re: [PATCH 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels |
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On Sun, Feb 4, 2018 at 9:42 AM, Dan Williams <dan.j.williams@intel.com> wrote: > On Sun, Feb 4, 2018 at 5:01 AM, Brian Gerst <brgerst@gmail.com> wrote: >> >> Now that the fast syscall path is gone, all regs (except RSP >> obviously) are dead after being saved to pt_regs. > > They're saved, but not dead afaics.
Actually, they _are_ dead with the slow-path - it will reload them off the ptregs pointer instead.
However, exactly because the slow-path will just reload the user-supplied values, there's no point in clearing them anyway. Those values will be live in the system call sequence anyway.
So even for the slow system call case, there's no point in clearing them. If we ever end up only reloading the required values (ie we push the 'struct ptregs' pointer lower down into the syscall stack), at that point we might want to clear those registers, because many system calls will reload only a small subset.
But as the commit message says, the system call argument registers are also likely to be aggressively clobbered unless used, since the low registers are preferred for code generation (smaller code, and many of them are special anyway in various ways and have forced uses for shifts, function arguments, or just are special in general like %rax).
So the actual argument registers tend to not be an issue anyway.
Linus
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