Re: [PATCH bpf-next 3/9] uprobes/x86: Finish an unoptimization which failed half-way

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Oct 22, 2026 at 01:57:06AM +0000, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> int3_update_unoptimize() restores optimized nop10 in three writes:
> 
>   48 8d 64 24 80 e8 d0 d1 d2 d3    lea -0x80(%rsp),%rsp; call tramp
>   cc 8d 64 24 80 e8 d0 d1 d2 d3    1. trap new entries
>   cc 2e 0f 1f 84 e8 d0 d1 d2 d3    2. restore bytes 1..4
>   66 2e 0f 1f 84 e8 d0 d1 d2 d3    3. restore byte 0
> 
> uprobe_write() can fail with -ENOMEM, or with -EINTR when the task is being
> killed. When the 2nd write fails the result of the 1st one stays. Byte 0 is
> a breakpoint and it works. But the next set_orig_insn() doesn't find lea in
> byte 0, takes it for a plain breakpoint and restores one byte:
> 
>   66 8d 64 24 80 e8 d0 d1 d2 d3    lea -0x80(%rsp),%sp; call tramp
> 
> Nothing recognizes these bytes anymore. The process keeps the call into
> uprobe trampoline, which can be unmapped already.
> 
> Teach is_optimized() to recognize the half-way state, and let the 1st write
> of int3_update_unoptimize() skip what is already done.
> 
> The failure wasn't reproduced, but the state can be made by hand. Attach to
> nop10, call it to get it optimized, write 0xcc over byte 0, detach.
> 
> "uprobes: Don't sleep on mmap_lock under dup_mmap_sem" needs this. There
> fork() removes the breakpoint from its mm. When fork() is killed in
> the middle register_for_each_vma() removes it for the second time.
> 
> Fixes: 554ba38456da ("uprobes/x86: Move optimized uprobe from nop5 to nop10")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

nice, great there's simple fix for that

Acked-by: Jiri Olsa <jolsa@kernel.org>

thanks,
jirka


> ---
>  arch/x86/kernel/uprobes.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 65a2de82ecd2..62bbe41c6466 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -896,6 +896,9 @@ static int verify_insn(struct page *page, unsigned long vaddr, uprobe_opcode_t *
>  	case EXPECT_OPTIMIZED:
>  		if (is_opt_insns(&old_opcode[0]))
>  			return 1;
> +		/* An earlier attempt got this far and failed, nothing to write. */
> +		if (is_swbp_opt_insns(&old_opcode[0]))
> +			return 0;
>  		break;
>  	case EXPECT_SWBP_OPTIMIZED:
>  		if (is_swbp_opt_insns(&old_opcode[0]))
> @@ -1109,6 +1112,17 @@ static bool __is_optimized(struct mm_struct *mm, uprobe_opcode_t *insn, unsigned
>  		s32 raddr;
>  	} *call = (struct __arch_relative_insn *)(insn + LEA_INSN_SIZE);
>  
> +	/*
> +	 * INT3 followed by the rest of the optimized instructions is what is
> +	 * left if int3_update_optimize() or int3_update_unoptimize() failed
> +	 * half-way. It works as a breakpoint, but to restore the original
> +	 * instruction it has to be unoptimized like a completely optimized one.
> +	 * The same has to be done if a debugger has put its INT3 on top of an
> +	 * optimized uprobe. The trampoline can be gone already, so do not look
> +	 * at the target of the call.
> +	 */
> +	if (is_swbp_opt_insns(insn))
> +		return true;
>  	if (!is_opt_insns(insn))
>  		return false;
>  	return __in_uprobe_trampoline(mm, vaddr + OPT_INSN_SIZE + call->raddr);
> -- 
> 2.55.0
>