[3/4] perf,x86: avoid missing caller address in stack traces captured in uprobe

Commit Message

Andrii Nakryiko May 8, 2024, 9:26 p.m. UTC

When tracing user functions with uprobe functionality, it's common to
install the probe (e.g., a BPF program) at the first instruction of the
function. This is often going to be `push %rbp` instruction in function
preamble, which means that within that function frame pointer hasn't
been established yet. This leads to consistently missing an actual
caller of the traced function, because perf_callchain_user() only
records current IP (capturing traced function) and then following frame
pointer chain (which would be caller's frame, containing the address of
caller's caller).

So when we have target_1 -> target_2 -> target_3 call chain and we are
tracing an entry to target_3, captured stack trace will report
target_1 -> target_3 call chain, which is wrong and confusing.

This patch proposes a x86-64-specific heuristic to detect `push %rbp`
instruction being traced. If that's the case, with the assumption that
applicatoin is compiled with frame pointers, this instruction would be
a strong indicator that this is the entry to the function. In that case,
return address is still pointed to by %rsp, so we fetch it and add to
stack trace before proceeding to unwind the rest using frame
pointer-based logic.

Signed-off-by: Andrii Nakryiko <[e-mail]>
---
 arch/x86/events/core.c  | 20 ++++++++++++++++++++
 include/linux/uprobes.h |  2 ++
 kernel/events/uprobes.c |  2 ++
 3 files changed, 24 insertions(+)

Comments

Jiri Olsa May 20, 2024, 3:20 p.m. UTC | #1

On Wed, May 08, 2024 at 02:26:04PM -0700, Andrii Nakryiko wrote:
> When tracing user functions with uprobe functionality, it's common to
> install the probe (e.g., a BPF program) at the first instruction of the
> function. This is often going to be `push %rbp` instruction in function
> preamble, which means that within that function frame pointer hasn't
> been established yet. This leads to consistently missing an actual
> caller of the traced function, because perf_callchain_user() only
> records current IP (capturing traced function) and then following frame
> pointer chain (which would be caller's frame, containing the address of
> caller's caller).
> 
> So when we have target_1 -> target_2 -> target_3 call chain and we are
> tracing an entry to target_3, captured stack trace will report
> target_1 -> target_3 call chain, which is wrong and confusing.
> 
> This patch proposes a x86-64-specific heuristic to detect `push %rbp`
> instruction being traced. If that's the case, with the assumption that
> applicatoin is compiled with frame pointers, this instruction would be
> a strong indicator that this is the entry to the function. In that case,
> return address is still pointed to by %rsp, so we fetch it and add to
> stack trace before proceeding to unwind the rest using frame
> pointer-based logic.
> 
> Signed-off-by: Andrii Nakryiko <[e-mail]>
> ---
>  arch/x86/events/core.c  | 20 ++++++++++++++++++++
>  include/linux/uprobes.h |  2 ++
>  kernel/events/uprobes.c |  2 ++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 5b0dd07b1ef1..82d5570b58ff 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2884,6 +2884,26 @@ perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs
>  		return;
>  
>  	pagefault_disable();
> +
> +#ifdef CONFIG_UPROBES
> +	/*
> +	 * If we are called from uprobe handler, and we are indeed at the very
> +	 * entry to user function (which is normally a `push %rbp` instruction,
> +	 * under assumption of application being compiled with frame pointers),
> +	 * we should read return address from *regs->sp before proceeding
> +	 * to follow frame pointers, otherwise we'll skip immediate caller
> +	 * as %rbp is not yet setup.
> +	 */
> +	if (current->utask) {
> +		struct arch_uprobe *auprobe = current->utask->auprobe;
> +		u64 ret_addr;
> +
> +		if (auprobe && auprobe->insn[0] == 0x55 /* push %rbp */ &&

nice cactch, I was wondering if we should set some auprobe flag in
arch_uprobe_analyze_insn and test it here instead of matching the
instruction opcode directly, but that would probably be just more
complicated.. this is simple and in arch code, no np

jirka

> +		    !__get_user(ret_addr, (const u64 __user *)regs->sp))
> +			perf_callchain_store(entry, ret_addr);
> +	}
> +#endif
> +
>  	while (entry->nr < entry->max_stack) {
>  		if (!valid_user_frame(fp, sizeof(frame)))
>  			break;
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index 0c57eec85339..7b785cd30d86 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -76,6 +76,8 @@ struct uprobe_task {
>  	struct uprobe			*active_uprobe;
>  	unsigned long			xol_vaddr;
>  
> +	struct arch_uprobe              *auprobe;
> +
>  	struct return_instance		*return_instances;
>  	unsigned int			depth;
>  };
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 1c99380dc89d..504693845187 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -2072,6 +2072,7 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  	bool need_prep = false; /* prepare return uprobe, when needed */
>  
>  	down_read(&uprobe->register_rwsem);
> +	current->utask->auprobe = &uprobe->arch;
>  	for (uc = uprobe->consumers; uc; uc = uc->next) {
>  		int rc = 0;
>  
> @@ -2086,6 +2087,7 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  
>  		remove &= rc;
>  	}
> +	current->utask->auprobe = NULL;
>  
>  	if (need_prep && !remove)
>  		prepare_uretprobe(uprobe, regs); /* put bp at return */
> -- 
> 2.43.0
> 
>

Patch

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 5b0dd07b1ef1..82d5570b58ff 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2884,6 +2884,26 @@  perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs
 		return;
 
 	pagefault_disable();
+
+#ifdef CONFIG_UPROBES
+	/*
+	 * If we are called from uprobe handler, and we are indeed at the very
+	 * entry to user function (which is normally a `push %rbp` instruction,
+	 * under assumption of application being compiled with frame pointers),
+	 * we should read return address from *regs->sp before proceeding
+	 * to follow frame pointers, otherwise we'll skip immediate caller
+	 * as %rbp is not yet setup.
+	 */
+	if (current->utask) {
+		struct arch_uprobe *auprobe = current->utask->auprobe;
+		u64 ret_addr;
+
+		if (auprobe && auprobe->insn[0] == 0x55 /* push %rbp */ &&
+		    !__get_user(ret_addr, (const u64 __user *)regs->sp))
+			perf_callchain_store(entry, ret_addr);
+	}
+#endif
+
 	while (entry->nr < entry->max_stack) {
 		if (!valid_user_frame(fp, sizeof(frame)))
 			break;
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index 0c57eec85339..7b785cd30d86 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -76,6 +76,8 @@  struct uprobe_task {
 	struct uprobe			*active_uprobe;
 	unsigned long			xol_vaddr;
 
+	struct arch_uprobe              *auprobe;
+
 	struct return_instance		*return_instances;
 	unsigned int			depth;
 };
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 1c99380dc89d..504693845187 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -2072,6 +2072,7 @@  static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
 	bool need_prep = false; /* prepare return uprobe, when needed */
 
 	down_read(&uprobe->register_rwsem);
+	current->utask->auprobe = &uprobe->arch;
 	for (uc = uprobe->consumers; uc; uc = uc->next) {
 		int rc = 0;
 
@@ -2086,6 +2087,7 @@  static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
 
 		remove &= rc;
 	}
+	current->utask->auprobe = NULL;
 
 	if (need_prep && !remove)
 		prepare_uretprobe(uprobe, regs); /* put bp at return */