Commit ae25884a authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

unwind_user/x86: Teach FP unwind about start of function 



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: default avatarJens Remus <jremus@linux.ibm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251024145156.GM4068168@noisy.programming.kicks-ass.net
parent 49cf34c0

arch/x86/events/core.c

+0 −40
Original line number Diff line number Diff line
@@ -2845,46 +2845,6 @@ static unsigned long get_segment_base(unsigned int segment) 
	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>

arch/x86/include/asm/unwind_user.h

+12 −0
Original line number Diff line number Diff line
@@ -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(struct pt_regs *regs) 
	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 */

arch/x86/include/asm/uprobes.h

+9 −0
Original line number Diff line number Diff line
@@ -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 */

arch/x86/kernel/uprobes.c

+32 −0
Original line number Diff line number Diff line
@@ -1791,3 +1791,35 @@ bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx, 
	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;

}

include/linux/unwind_user_types.h

+1 −0
Original line number Diff line number Diff line
@@ -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;

};