Messages in this thread | | | Date | Fri, 15 Oct 2021 18:56:35 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support |
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On Fri, Oct 15, 2021 at 04:24:08PM +0200, Borislav Petkov wrote: > On Wed, Oct 13, 2021 at 02:22:21PM +0200, Peter Zijlstra wrote: > > +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes) > > +{ > > + void (*target)(void); > > + int reg, i = 0; > > + > > + if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) > > + return -1; > > + > > + target = addr + insn->length + insn->immediate.value; > > + reg = (target - &__x86_indirect_thunk_rax) / > > + (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax); > > I guess you should compute those values once so that it doesn't have to > do them for each function invocation. And it does them here when I look > at the asm it generates.
Takes away the simplicity of the thing. It can't know these values at compile time (due to external symbols etc..) although I suppose LTO might be able to fix that.
Other than that, the above is the trivial form of reverse indexing an array.
> > + > > + if (WARN_ON_ONCE(reg & ~0xf)) > > + return -1; > > Sanity-checking the alignment of those thunks?
Nah, the target address of the instruction; if that's not a retpoline thunk (for whatever raisin) then the computation will not result in a valid reg and we should bail.
> > + > > + i = emit_indirect(insn->opcode.bytes[0], reg, bytes); > > + if (i < 0) > > + return i; > > + > > + for (; i < insn->length;) > > + bytes[i++] = BYTES_NOP1; > > Why not: > > nop_len = insn->length - i; > if (nop_len) { > memcpy(&bytes[i], x86_nops[nop_len], nop_len); > i += nop_len; > } > > and then you save yourself the optimize_nops() call because it'll take > the right-sized NOP directly.
That's not immediately safe; if for some reason or other the original instrucion is 15 bytes long, and we generated 2 bytes, then we need 13 nop bytes, the above will then do an out-of-bound array access (due to the nops array only doing 8 byte nops at max).
I wanted this code to be simple and obvious.
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