Re: [PATCH v22 19/20] ftrace: Add ftrace_get_symaddr to convert fentry_ip to symaddr

[Gabriel de Perthuis <g2p.code@gmail.com>]

Hello,

I got errors building Linux 6.14-rc1 that were solved by reverting this 
patch and the one after (19/20 and 20/20).

Errors look like:

In file included from ./arch/x86/include/asm/asm-prototypes.h:2,
                  from <stdin>:3:
./arch/x86/include/asm/ftrace.h: In function 'arch_ftrace_get_symaddr':
./arch/x86/include/asm/ftrace.h:46:21: error: implicit declaration of 
function 'get_kernel_nofault' [-Wimplicit-function-declaration]
    46 |                 if (get_kernel_nofault(instr, (u32 *)(fentry_ip 
- ENDBR_INSN_SIZE)))
       | ^~~~~~~~~~~~~~~~~~

Will send .config on request if needed.

Le 26/12/2024 à 06:15, Masami Hiramatsu (Google) a écrit :
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
>
> This introduces ftrace_get_symaddr() which tries to convert fentry_ip
> passed by ftrace or fgraph callback to symaddr without calling
> kallsyms API. It returns the symbol address or 0 if it fails to
> convert it.
>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202412061423.K79V55Hd-lkp@intel.com/
> Closes: https://lore.kernel.org/oe-kbuild-all/202412061804.5VRzF14E-lkp@intel.com/
> ---
>   Changes in v21:
>    - On arm64, fix the macro name to ftrace_get_symaddr() correctly.
>    - Define ftrace_get_symaddr() outside of CONFIG_DYNAMIC_FTRACE.
>   Changes in v19:
>    - Newly added.
> ---
>   arch/arm64/include/asm/ftrace.h |    2 +
>   arch/arm64/kernel/ftrace.c      |   63 +++++++++++++++++++++++++++++++++++++++
>   arch/x86/include/asm/ftrace.h   |   21 +++++++++++++
>   include/linux/ftrace.h          |   13 ++++++++
>   4 files changed, 99 insertions(+)
>
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index 876e88ad4119..bfe3ce9df197 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -52,6 +52,8 @@ extern unsigned long ftrace_graph_call;
>   extern void return_to_handler(void);
>   
>   unsigned long ftrace_call_adjust(unsigned long addr);
> +unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip);
> +#define ftrace_get_symaddr(fentry_ip) arch_ftrace_get_symaddr(fentry_ip)
>   
>   #ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
>   #define HAVE_ARCH_FTRACE_REGS
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 570c38be833c..d7c0d023dfe5 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -143,6 +143,69 @@ unsigned long ftrace_call_adjust(unsigned long addr)
>   	return addr;
>   }
>   
> +/* Convert fentry_ip to the symbol address without kallsyms */
> +unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip)
> +{
> +	u32 insn;
> +
> +	/*
> +	 * When using patchable-function-entry without pre-function NOPS, ftrace
> +	 * entry is the address of the first NOP after the function entry point.
> +	 *
> +	 * The compiler has either generated:
> +	 *
> +	 * func+00:	func:	NOP		// To be patched to MOV X9, LR
> +	 * func+04:		NOP		// To be patched to BL <caller>
> +	 *
> +	 * Or:
> +	 *
> +	 * func-04:		BTI	C
> +	 * func+00:	func:	NOP		// To be patched to MOV X9, LR
> +	 * func+04:		NOP		// To be patched to BL <caller>
> +	 *
> +	 * The fentry_ip is the address of `BL <caller>` which is at `func + 4`
> +	 * bytes in either case.
> +	 */
> +	if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS))
> +		return fentry_ip - AARCH64_INSN_SIZE;
> +
> +	/*
> +	 * When using patchable-function-entry with pre-function NOPs, BTI is
> +	 * a bit different.
> +	 *
> +	 * func+00:	func:	NOP		// To be patched to MOV X9, LR
> +	 * func+04:		NOP		// To be patched to BL <caller>
> +	 *
> +	 * Or:
> +	 *
> +	 * func+00:	func:	BTI	C
> +	 * func+04:		NOP		// To be patched to MOV X9, LR
> +	 * func+08:		NOP		// To be patched to BL <caller>
> +	 *
> +	 * The fentry_ip is the address of `BL <caller>` which is at either
> +	 * `func + 4` or `func + 8` depends on whether there is a BTI.
> +	 */
> +
> +	/* If there is no BTI, the func address should be one instruction before. */
> +	if (!IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> +		return fentry_ip - AARCH64_INSN_SIZE;
> +
> +	/* We want to be extra safe in case entry ip is on the page edge,
> +	 * but otherwise we need to avoid get_kernel_nofault()'s overhead.
> +	 */
> +	if ((fentry_ip & ~PAGE_MASK) < AARCH64_INSN_SIZE * 2) {
> +		if (get_kernel_nofault(insn, (u32 *)(fentry_ip - AARCH64_INSN_SIZE * 2)))
> +			return 0;
> +	} else {
> +		insn = *(u32 *)(fentry_ip - AARCH64_INSN_SIZE * 2);
> +	}
> +
> +	if (aarch64_insn_is_bti(le32_to_cpu((__le32)insn)))
> +		return fentry_ip - AARCH64_INSN_SIZE * 2;
> +
> +	return fentry_ip - AARCH64_INSN_SIZE;
> +}
> +
>   /*
>    * Replace a single instruction, which may be a branch or NOP.
>    * If @validate == true, a replaced instruction is checked against 'old'.
> diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> index cc92c99ef276..f9cb4d07df58 100644
> --- a/arch/x86/include/asm/ftrace.h
> +++ b/arch/x86/include/asm/ftrace.h
> @@ -34,6 +34,27 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
>   	return addr;
>   }
>   
> +static inline unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip)
> +{
> +#ifdef CONFIG_X86_KERNEL_IBT
> +	u32 instr;
> +
> +	/* We want to be extra safe in case entry ip is on the page edge,
> +	 * but otherwise we need to avoid get_kernel_nofault()'s overhead.
> +	 */
> +	if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) {
> +		if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE)))
> +			return fentry_ip;
> +	} else {
> +		instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE);
> +	}
> +	if (is_endbr(instr))
> +		fentry_ip -= ENDBR_INSN_SIZE;
> +#endif
> +	return fentry_ip;
> +}
> +#define ftrace_get_symaddr(fentry_ip)	arch_ftrace_get_symaddr(fentry_ip)
> +
>   #ifdef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
>   
>   #include <linux/ftrace_regs.h>
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 4c553fe9c026..07092dfb21a4 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -622,6 +622,19 @@ enum {
>   	FTRACE_MAY_SLEEP		= (1 << 5),
>   };
>   
> +/* Arches can override ftrace_get_symaddr() to convert fentry_ip to symaddr. */
> +#ifndef ftrace_get_symaddr
> +/**
> + * ftrace_get_symaddr - return the symbol address from fentry_ip
> + * @fentry_ip: the address of ftrace location
> + *
> + * Get the symbol address from @fentry_ip (fast path). If there is no fast
> + * search path, this returns 0.
> + * User may need to use kallsyms API to find the symbol address.
> + */
> +#define ftrace_get_symaddr(fentry_ip) (0)
> +#endif
> +
>   #ifdef CONFIG_DYNAMIC_FTRACE
>   
>   void ftrace_arch_code_modify_prepare(void);
>
>