Re: [PATCH RFC bpf-next 1/7] ftrace: introduce FTRACE_OPS_FL_JMP

bot+bpf-ci
2025-11-14T10:20:26Z

[-- Attachment #1: Type: text/plain, Size: 3431 bytes --]

> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 7ded7df6e..14705dec1 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -351,6 +351,7 @@ enum {
> FTRACE_OPS_FL_DIRECT = BIT(17),
> FTRACE_OPS_FL_SUBOP = BIT(18),
> FTRACE_OPS_FL_GRAPH = BIT(19),
> + FTRACE_OPS_FL_JMP = BIT(20),
> };
>
> #ifndef CONFIG_DYNAMIC_FTRACE_WITH_ARGS

[ ... ]

> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_JMP
> +static inline bool ftrace_is_jmp(unsigned long addr)
> +{
> + return addr & 1;
> +}
> +
> +static inline unsigned long ftrace_jmp_set(unsigned long addr)
> +{
> + return addr | 1UL;
> +}
> +
> +static inline unsigned long ftrace_jmp_get(unsigned long addr)
> +{
> + return addr & ~1UL;
> +}

[ ... ]

> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 42bd2ba68..b143a79e6 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5938,7 +5938,8 @@ static void remove_direct_functions_hash(struct ftrace_hash *hash, unsigned long
> for (i = 0; i < size; i++) {
> hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> del = __ftrace_lookup_ip(direct_functions, entry->ip);
> - if (del && del->direct == addr) {
> + if (del && ftrace_jmp_get(del->direct) ==
> + ftrace_jmp_get(addr)) {
> remove_hash_entry(direct_functions, del);
> kfree(del);
> }

[ ... ]

> @@ -5994,6 +5995,9 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>
> mutex_lock(&direct_mutex);
>
> + if (ops->flags & FTRACE_OPS_FL_JMP)
> + addr = ftrace_jmp_set(addr);
> +

When FTRACE_OPS_FL_JMP is set, this encodes the flag into the last bit
of addr. The modified addr is then stored in entry->direct and
ops->direct_call. However, this flagged address later flows through
ftrace_find_rec_direct()->ftrace_get_addr_new()->ftrace_make_call()->
ftrace_call_replace() without being masked.

Can ftrace_call_replace() generate correct CALL instructions when addr
has the LSB set? On x86, ftrace_call_replace() calls text_gen_insn()
with the raw address to generate a relative CALL instruction. If addr
has bit 0 set to 1, this would generate a CALL to an odd address,
causing execution to skip the first instruction byte at the trampoline.

Similarly, at runtime call_direct_funcs() reads ops->direct_call and
passes it to arch_ftrace_set_direct_caller() without masking. The x86
assembly then does RET to this address, which would jump to addr+1 if
the LSB is set.

The commit message says "we can tell if we should use 'jmp' for the
callback in ftrace_call_replace()" but ftrace_call_replace() isn't
modified to check or mask the bit. Are there missing changes to handle
the encoded address in these paths?

> /* Make sure requested entries are not already registered.. */
> size = 1 << hash->size_bits;
> for (i = 0; i < size; i++) {

[ ... ]

> @@ -6117,6 +6121,9 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>
> lockdep_assert_held_once(&direct_mutex);
>
> + if (ops->flags & FTRACE_OPS_FL_JMP)
> + addr = ftrace_jmp_set(addr);
> +

Same issue here - the flagged addr is stored but downstream consumers
don't mask it before using as a jump target.

---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/19360353328