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Date: Thu, 3 Mar 2022 11:53:44 +0100 (CET)
From: Miroslav Benes <mbenes@...e.cz>
To: Peter Zijlstra <peterz@...radead.org>
cc: x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
jpoimboe@...hat.com, andrew.cooper3@...rix.com,
linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
keescook@...omium.org, samitolvanen@...gle.com,
mark.rutland@....com, alyssa.milburn@...el.com,
rostedt@...dmis.org, mhiramat@...nel.org,
alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 33/39] objtool: Add IBT/ENDBR decoding
Hi,
On Thu, 24 Feb 2022, Peter Zijlstra wrote:
> Decode ENDBR instructions and WARN about NOTRACK prefixes.
I guess it has been already mentioned somewhere, but could you explain
NOTRACK prefix here, please? If I understand it right, it disables IBT for
the indirect branch instruction meaning that its target does not have to
start with ENDBR?
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> tools/objtool/arch/x86/decode.c | 34 +++++++++++++++++++++++++++++-----
> tools/objtool/include/objtool/arch.h | 1 +
> 2 files changed, 30 insertions(+), 5 deletions(-)
>
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -103,6 +103,18 @@ unsigned long arch_jump_destination(stru
> #define rm_is_mem(reg) (mod_is_mem() && !is_RIP() && rm_is(reg))
> #define rm_is_reg(reg) (mod_is_reg() && modrm_rm == (reg))
>
> +static bool has_notrack_prefix(struct insn *insn)
> +{
> + int i;
> +
> + for (i = 0; i < insn->prefixes.nbytes; i++) {
> + if (insn->prefixes.bytes[i] == 0x3e)
> + return true;
> + }
> +
> + return false;
> +}
> +
...
> @@ -636,20 +656,24 @@ int arch_decode_instruction(struct objto
> break;
>
> case 0xff:
> - if (modrm_reg == 2 || modrm_reg == 3)
> + if (modrm_reg == 2 || modrm_reg == 3) {
>
> *type = INSN_CALL_DYNAMIC;
> + if (has_notrack_prefix(&insn))
> + WARN("notrack prefix found at %s:0x%lx", sec->name, offset);
>
> - else if (modrm_reg == 4)
> + } else if (modrm_reg == 4) {
>
> *type = INSN_JUMP_DYNAMIC;
> + if (has_notrack_prefix(&insn))
> + WARN("notrack prefix found at %s:0x%lx", sec->name, offset);
And we want to warn about it here so that we can have it all in the kernel
control?
Miroslav
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