Date: Sun, 14 Jun 2020 19:26:07 +1000 From: Nicholas Piggin <npiggin@...il.com> To: Segher Boessenkool <segher@...nel.crashing.org> Cc: libc-dev@...ts.llvm.org, linux-api@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, musl@...ts.openwall.com Subject: Re: Linux powerpc new system call instruction and ABI Excerpts from Segher Boessenkool's message of June 12, 2020 7:02 am: > Hi! > > On Thu, Jun 11, 2020 at 06:12:01PM +1000, Nicholas Piggin wrote: >> Calling convention >> ------------------ >> The proposal is for scv 0 to provide the standard Linux system call ABI >> with the following differences from sc convention: >> >> - lr is to be volatile across scv calls. This is necessary because the >> scv instruction clobbers lr. From previous discussion, this should be >> possible to deal with in GCC clobbers and CFI. >> >> - cr1 and cr5-cr7 are volatile. This matches the C ABI and would allow the >> kernel system call exit to avoid restoring the volatile cr registers >> (although we probably still would anyway to avoid information leaks). >> >> - Error handling: The consensus among kernel, glibc, and musl is to move to >> using negative return values in r3 rather than CR0[SO]=1 to indicate error, >> which matches most other architectures, and is closer to a function call. > > What about cr0 then? Will it be volatile as well (exactly like for > function calls)? Yes, same as for sc (except for SO bit). Which is a bit unclear in this section. >> Notes >> ----- >> - r0,r4-r8 are documented as volatile in the ABI, but the kernel patch as >> submitted currently preserves them. This is to leave room for deciding >> which way to go with these. > > The kernel has to set it to *something* that doesn't leak information ;-) For "sc" system calls these were defined as volatile (and used to just leak information), so now we just zero them. Thanks, Nick
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