Messages in this thread | | | Date | Mon, 18 Oct 2021 23:06:35 +0000 | From | Alexander Lobakin <> | Subject | Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support |
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From: Peter Zijlstra <peterz@infradead.org> Date: Fri, 15 Oct 2021 18:56:35 +0200
Hi,
Gave it a spin with Clang/LLVM, and
> 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);
this triggers
> > 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;
this:
WARN in patch_retpoline:408: addr pcibios_scan_specific_bus+0x196/0x200, op 0xe8, reg 0xb88ca WARN in patch_retpoline:408: addr xen_pv_teardown_msi_irqs+0x8d/0x120, op 0xe8, reg 0xb88ca WARN in patch_retpoline:408: addr __mptcp_sockopt_sync+0x7e/0x200, op 0xe8, reg 0xb88ca [...] (thousands of them, but op == 0xe8 && reg == 0xb88ca are always the same)
I know this reg calculation is about to be replaced, but anyway ;)
> > 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.
Thanks, Al
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