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SubjectRe: [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@infradead.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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