On Fri, Oct 24, 2025 at 04:08:15PM +0200, Peter Zijlstra wrote: > Yeah, I suppose that should work. Let me rework things accordingly.
--- Subject: unwind_user/x86: Teach FP unwind about start of function From: Peter Zijlstra <[email protected]> Date: Fri Oct 24 12:31:10 CEST 2025 When userspace is interrupted at the start of a function, before we get a chance to complete the frame, unwind will miss one caller. X86 has a uprobe specific fixup for this, add bits to the generic unwinder to support this. Suggested-by: Jens Remus <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> --- arch/x86/events/core.c | 40 ------------------------------------- arch/x86/include/asm/unwind_user.h | 12 +++++++++++ arch/x86/include/asm/uprobes.h | 9 ++++++++ arch/x86/kernel/uprobes.c | 32 +++++++++++++++++++++++++++++ include/linux/unwind_user_types.h | 1 kernel/unwind/user.c | 35 ++++++++++++++++++++++++-------- 6 files changed, 80 insertions(+), 49 deletions(-) --- a/arch/x86/events/core.c +++ b/arch/x86/events/core.c @@ -2845,46 +2845,6 @@ static unsigned long get_segment_base(un return get_desc_base(desc); } -#ifdef CONFIG_UPROBES -/* - * Heuristic-based check if uprobe is installed at the function entry. - * - * Under assumption of user code being compiled with frame pointers, - * `push %rbp/%ebp` is a good indicator that we indeed are. - * - * Similarly, `endbr64` (assuming 64-bit mode) is also a common pattern. - * If we get this wrong, captured stack trace might have one extra bogus - * entry, but the rest of stack trace will still be meaningful. - */ -static bool is_uprobe_at_func_entry(struct pt_regs *regs) -{ - struct arch_uprobe *auprobe; - - if (!current->utask) - return false; - - auprobe = current->utask->auprobe; - if (!auprobe) - return false; - - /* push %rbp/%ebp */ - if (auprobe->insn[0] == 0x55) - return true; - - /* endbr64 (64-bit only) */ - if (user_64bit_mode(regs) && is_endbr((u32 *)auprobe->insn)) - return true; - - return false; -} - -#else -static bool is_uprobe_at_func_entry(struct pt_regs *regs) -{ - return false; -} -#endif /* CONFIG_UPROBES */ - #ifdef CONFIG_IA32_EMULATION #include <linux/compat.h> --- a/arch/x86/include/asm/unwind_user.h +++ b/arch/x86/include/asm/unwind_user.h @@ -3,6 +3,7 @@ #define _ASM_X86_UNWIND_USER_H #include <asm/ptrace.h> +#include <asm/uprobes.h> #define ARCH_INIT_USER_FP_FRAME(ws) \ .cfa_off = 2*(ws), \ @@ -10,6 +11,12 @@ .fp_off = -2*(ws), \ .use_fp = true, +#define ARCH_INIT_USER_FP_ENTRY_FRAME(ws) \ + .cfa_off = 1*(ws), \ + .ra_off = -1*(ws), \ + .fp_off = 0, \ + .use_fp = false, + static inline int unwind_user_word_size(struct pt_regs *regs) { /* We can't unwind VM86 stacks */ @@ -22,4 +29,9 @@ static inline int unwind_user_word_size( return sizeof(long); } +static inline bool unwind_user_at_function_start(struct pt_regs *regs) +{ + return is_uprobe_at_func_entry(regs); +} + #endif /* _ASM_X86_UNWIND_USER_H */ --- a/arch/x86/include/asm/uprobes.h +++ b/arch/x86/include/asm/uprobes.h @@ -62,4 +62,13 @@ struct arch_uprobe_task { unsigned int saved_tf; }; +#ifdef CONFIG_UPROBES +extern bool is_uprobe_at_func_entry(struct pt_regs *regs); +#else +static bool is_uprobe_at_func_entry(struct pt_regs *regs) +{ + return false; +} +#endif /* CONFIG_UPROBES */ + #endif /* _ASM_UPROBES_H */ --- a/arch/x86/kernel/uprobes.c +++ b/arch/x86/kernel/uprobes.c @@ -1791,3 +1791,35 @@ bool arch_uretprobe_is_alive(struct retu else return regs->sp <= ret->stack; } + +/* + * Heuristic-based check if uprobe is installed at the function entry. + * + * Under assumption of user code being compiled with frame pointers, + * `push %rbp/%ebp` is a good indicator that we indeed are. + * + * Similarly, `endbr64` (assuming 64-bit mode) is also a common pattern. + * If we get this wrong, captured stack trace might have one extra bogus + * entry, but the rest of stack trace will still be meaningful. + */ +bool is_uprobe_at_func_entry(struct pt_regs *regs) +{ + struct arch_uprobe *auprobe; + + if (!current->utask) + return false; + + auprobe = current->utask->auprobe; + if (!auprobe) + return false; + + /* push %rbp/%ebp */ + if (auprobe->insn[0] == 0x55) + return true; + + /* endbr64 (64-bit only) */ + if (user_64bit_mode(regs) && is_endbr((u32 *)auprobe->insn)) + return true; + + return false; +} --- a/include/linux/unwind_user_types.h +++ b/include/linux/unwind_user_types.h @@ -39,6 +39,7 @@ struct unwind_user_state { unsigned int ws; enum unwind_user_type current_type; unsigned int available_types; + bool topmost; bool done; }; --- a/kernel/unwind/user.c +++ b/kernel/unwind/user.c @@ -26,14 +26,12 @@ get_user_word(unsigned long *word, unsig return get_user(*word, addr); } -static int unwind_user_next_fp(struct unwind_user_state *state) +static int unwind_user_next_common(struct unwind_user_state *state, + const struct unwind_user_frame *frame) { - const struct unwind_user_frame frame = { - ARCH_INIT_USER_FP_FRAME(state->ws) - }; unsigned long cfa, fp, ra; - if (frame.use_fp) { + if (frame->use_fp) { if (state->fp < state->sp) return -EINVAL; cfa = state->fp; @@ -42,7 +40,7 @@ static int unwind_user_next_fp(struct un } /* Get the Canonical Frame Address (CFA) */ - cfa += frame.cfa_off; + cfa += frame->cfa_off; /* stack going in wrong direction? */ if (cfa <= state->sp) @@ -53,19 +51,37 @@ static int unwind_user_next_fp(struct un return -EINVAL; /* Find the Return Address (RA) */ - if (get_user_word(&ra, cfa, frame.ra_off, state->ws)) + if (get_user_word(&ra, cfa, frame->ra_off, state->ws)) return -EINVAL; - if (frame.fp_off && get_user_word(&fp, cfa, frame.fp_off, state->ws)) + if (frame->fp_off && get_user_word(&fp, cfa, frame->fp_off, state->ws)) return -EINVAL; state->ip = ra; state->sp = cfa; - if (frame.fp_off) + if (frame->fp_off) state->fp = fp; + state->topmost = false; return 0; } +static int unwind_user_next_fp(struct unwind_user_state *state) +{ + struct pt_regs *regs = task_pt_regs(current); + + const struct unwind_user_frame fp_frame = { + ARCH_INIT_USER_FP_FRAME(state->ws) + }; + const struct unwind_user_frame fp_entry_frame = { + ARCH_INIT_USER_FP_ENTRY_FRAME(state->ws) + }; + + if (state->topmost && unwind_user_at_function_start(regs)) + return unwind_user_next_common(state, &fp_entry_frame); + + return unwind_user_next_common(state, &fp_frame); +} + static int unwind_user_next(struct unwind_user_state *state) { unsigned long iter_mask = state->available_types; @@ -118,6 +134,7 @@ static int unwind_user_start(struct unwi state->done = true; return -EINVAL; } + state->topmost = true; return 0; }
