Messages in this thread | | | Date | Wed, 2 Jun 2021 10:14:58 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/alternative: Optimize single-byte NOPs at an arbitrary position |
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On Tue, Jun 01, 2021 at 11:21:25PM +0200, Borislav Petkov wrote: > From: Borislav Petkov <bp@suse.de> > > Up until now the assumption was that an alternative patching site would > have some instructions at the beginning and trailing single-byte NOPs > (0x90) padding. Therefore, the patching machinery would go and optimize > those single-byte NOPs into longer ones. > > However, this assumption is broken on 32-bit when code like > hv_do_hypercall() in hyperv_init() would use the ratpoline speculation > killer CALL_NOSPEC. The 32-bit version of that macro would align certain > insns to 16 bytes, leading to the compiler issuing a one or more > single-byte NOPs, depending on the holes it needs to fill for alignment.
Who again insisted that wouldn't happen? :-)
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c > index 6974b5174495..7baf13b11952 100644 > --- a/arch/x86/kernel/alternative.c > +++ b/arch/x86/kernel/alternative.c > @@ -182,42 +182,68 @@ recompute_jump(struct alt_instr *a, u8 *orig_insn, u8 *repl_insn, u8 *insn_buff) > n_dspl, (unsigned long)orig_insn + n_dspl + repl_len); > } > > +/* > + * @instr: instruction byte stream > + * @instrlen: length of the above > + * @off: offset within @instr where the first NOP has been detected > + */
That's almost a proper comment, might as well finish it
/* * optimize_nops_range() - Optimize a sequence of single byte NOPs (0x90) * @instr: instruction byte stream * @instrlen: length of the above * @off: offset within @instr where the first NOP has been detected * * Return: number of NOPs found (and replaced) */ > +static __always_inline int optimize_nops_range(u8 *instr, u8 instrlen, int off) > +{ > + unsigned long flags; > + int i = off, nnops; > + > + while (i < instrlen) { > + if (instr[i] != 0x90) > + break; > + > + i++; > + }
for (; i < instrlen && instr[i] == 0x90; i++) ;
perhaps? (possibly too dense, up to you)
> + > + nnops = i - off; > + > + if (nnops <= 1) > + return nnops;
!nnops would be an error, no?
> + > + local_irq_save(flags); > + add_nops(instr + off, nnops); > + local_irq_restore(flags); > + > + DUMP_BYTES(instr, instrlen, "%px: [%d:%d) optimized NOPs: ", > + instr, off, i); > + > + return nnops; > +} > + > +
We really needs that extra line?
> /* > * "noinline" to cause control flow change and thus invalidate I$ and > * cause refetch after modification. > */ > static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr) > { > struct insn insn; > + int i = 0; > > /* > + * Jump over the non-NOP insns and optimize single-byte NOPs into bigger > + * ones. > */ > for (;;) { > if (insn_decode_kernel(&insn, &instr[i])) > return; > > + /* > + * See if this and any potentially following NOPs can be > + * optimized. > + */ > if (insn.length == 1 && insn.opcode.bytes[0] == 0x90) > + i += optimize_nops_range(instr, a->instrlen, i); > + else > + i += insn.length; > > + if (i >= a->instrlen) > return; > } > }
Anyway, irrespective of nits:
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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