When adding a kprobe such as "p:probe/tcp_sendmsg _text+15392192",
arch_check_kprobe would start iterating all instructions starting from
_text until the probed address. Not only is this very inefficient, but
literal values in there (e.g. left by function patching) are
misinterpreted in a way that causes a desync.

Fix this by doing it like x86: start the iteration at the closest
preceding symbol instead of the given starting point.

Fixes: 87f48c7ccc73 ("riscv: kprobe: Fixup kernel panic when probing an illegal position")
Signed-off-by: Fabian Vogt <fvogt@suse.de>
Signed-off-by: Marvin Friedrich <marvin.friedrich@suse.com>
---
 arch/riscv/kernel/probes/kprobes.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index c0738d6c6498..8723390c7cad 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -49,10 +49,15 @@ static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
 	post_kprobe_handler(p, kcb, regs);
 }
 
-static bool __kprobes arch_check_kprobe(struct kprobe *p)
+static bool __kprobes arch_check_kprobe(unsigned long addr)
 {
-	unsigned long tmp  = (unsigned long)p->addr - p->offset;
-	unsigned long addr = (unsigned long)p->addr;
+	unsigned long tmp, offset;
+
+	/* start iterating at the closest preceding symbol */
+	if (!kallsyms_lookup_size_offset(addr, NULL, &offset))
+		return false;
+
+	tmp = addr - offset;
 
 	while (tmp <= addr) {
 		if (tmp == addr)
@@ -71,7 +76,7 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 	if ((unsigned long)insn & 0x1)
 		return -EILSEQ;
 
-	if (!arch_check_kprobe(p))
+	if (!arch_check_kprobe((unsigned long)p->addr))
 		return -EILSEQ;
 
 	/* copy instruction */
-- 
2.51.0

On Wed, Sep 10, 2025 at 11:25 PM Fabian Vogt <fvogt@suse.de> wrote:
>
> When adding a kprobe such as "p:probe/tcp_sendmsg _text+15392192",
> arch_check_kprobe would start iterating all instructions starting from
> _text until the probed address. Not only is this very inefficient, but
> literal values in there (e.g. left by function patching) are
> misinterpreted in a way that causes a desync.

LGTM! This prevents bad p->offset from causing problems.

Acked-by: Guo Ren <guoren@kernel.org>

>
> Fix this by doing it like x86: start the iteration at the closest
> preceding symbol instead of the given starting point.
>
> Fixes: 87f48c7ccc73 ("riscv: kprobe: Fixup kernel panic when probing an illegal position")
> Signed-off-by: Fabian Vogt <fvogt@suse.de>
> Signed-off-by: Marvin Friedrich <marvin.friedrich@suse.com>
> ---
>  arch/riscv/kernel/probes/kprobes.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> index c0738d6c6498..8723390c7cad 100644
> --- a/arch/riscv/kernel/probes/kprobes.c
> +++ b/arch/riscv/kernel/probes/kprobes.c
> @@ -49,10 +49,15 @@ static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
>         post_kprobe_handler(p, kcb, regs);
>  }
>
> -static bool __kprobes arch_check_kprobe(struct kprobe *p)
> +static bool __kprobes arch_check_kprobe(unsigned long addr)
>  {
> -       unsigned long tmp  = (unsigned long)p->addr - p->offset;
> -       unsigned long addr = (unsigned long)p->addr;
> +       unsigned long tmp, offset;
> +
> +       /* start iterating at the closest preceding symbol */
> +       if (!kallsyms_lookup_size_offset(addr, NULL, &offset))
> +               return false;
> +
> +       tmp = addr - offset;
>
>         while (tmp <= addr) {
>                 if (tmp == addr)
> @@ -71,7 +76,7 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
>         if ((unsigned long)insn & 0x1)
>                 return -EILSEQ;
>
> -       if (!arch_check_kprobe(p))
> +       if (!arch_check_kprobe((unsigned long)p->addr))
>                 return -EILSEQ;
>
>         /* copy instruction */
> --
> 2.51.0
>
>
>
>


-- 
Best Regards
 Guo Ren