Re: [PATCH v2] arm64: ftrace: fix unreachable PLT for ftrace_caller in init_module with CONFIG_DYNAMIC_FTRACE

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Sep 04, 2025 at 08:22:36PM -0700, panfan wrote:
> On arm64, it has been possible for a module's sections to be placed more
> than 128M away from each other since commit:
> 
>   commit 3e35d303ab7d ("arm64: module: rework module VA range selection")
> 
> Due to this, an ftrace callsite in a module's .init.text section can be
> out of branch range for the module's ftrace PLT entry (in the module's
> .text section). Any attempt to enable tracing of that callsite will
> result in a BRK being patched into the callsite, resulting in a fatal
> exception when the callsite is later executed.
> 
> Fix this by adding an additional trampoline for .init.text, which will
> be within range.
> 
> No additional trampolines are necessary due to the way a given
> module's executable sections are packed together. Any executable
> section beginning with ".init" will be placed in MOD_INIT_TEXT,
> and any other executable section, including those beginning with ".exit",
>  will be placed in MOD_TEXT.
> 
> Fixes: 3e35d303ab7d ("arm64: module: rework module VA range selection")
> 
> Signed-off-by: panfan <panfan@qti.qualcomm.com>

Thanks for respinning this! This looks good to me, so:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Catalin, Will, I assume you'll add a CC stable when picking this up.

Mark.

> ---
> v2:
>    - Add an explicit check to ensure the address is within MOD_INIT_TEXT
>      or MOD_TEXT.
>    - Simplify the commit message.
> 
> Link: https://lore.kernel.org/all/20250819063418.1263119-1-panfan@qti.qualcomm.com/   # v1
> ---
>  arch/arm64/include/asm/module.h     |  1 +
>  arch/arm64/include/asm/module.lds.h |  1 +
>  arch/arm64/kernel/ftrace.c          | 13 ++++++++++---
>  arch/arm64/kernel/module-plts.c     | 12 +++++++++++-
>  arch/arm64/kernel/module.c          | 11 +++++++++++
>  5 files changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/module.h b/arch/arm64/include/asm/module.h
> index 79550b22ba19..fb9b88eebeb1 100644
> --- a/arch/arm64/include/asm/module.h
> +++ b/arch/arm64/include/asm/module.h
> @@ -19,6 +19,7 @@ struct mod_arch_specific {
>  
>  	/* for CONFIG_DYNAMIC_FTRACE */
>  	struct plt_entry	*ftrace_trampolines;
> +	struct plt_entry	*init_ftrace_trampolines;
>  };
>  
>  u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
> diff --git a/arch/arm64/include/asm/module.lds.h b/arch/arm64/include/asm/module.lds.h
> index b9ae8349e35d..fb944b46846d 100644
> --- a/arch/arm64/include/asm/module.lds.h
> +++ b/arch/arm64/include/asm/module.lds.h
> @@ -2,6 +2,7 @@ SECTIONS {
>  	.plt 0 : { BYTE(0) }
>  	.init.plt 0 : { BYTE(0) }
>  	.text.ftrace_trampoline 0 : { BYTE(0) }
> +	.init.text.ftrace_trampoline 0 : { BYTE(0) }
>  
>  #ifdef CONFIG_KASAN_SW_TAGS
>  	/*
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 5a890714ee2e..5adad37ab4fa 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -258,10 +258,17 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
>  	return ftrace_modify_code(pc, 0, new, false);
>  }
>  
> -static struct plt_entry *get_ftrace_plt(struct module *mod)
> +static struct plt_entry *get_ftrace_plt(struct module *mod, unsigned long addr)
>  {
>  #ifdef CONFIG_MODULES
> -	struct plt_entry *plt = mod->arch.ftrace_trampolines;
> +	struct plt_entry *plt = NULL;
> +
> +	if (within_module_mem_type(addr, mod, MOD_INIT_TEXT))
> +		plt = mod->arch.init_ftrace_trampolines;
> +	else if (within_module_mem_type(addr, mod, MOD_TEXT))
> +		plt = mod->arch.ftrace_trampolines;
> +	else
> +		return NULL;
>  
>  	return &plt[FTRACE_PLT_IDX];
>  #else
> @@ -332,7 +339,7 @@ static bool ftrace_find_callable_addr(struct dyn_ftrace *rec,
>  	if (WARN_ON(!mod))
>  		return false;
>  
> -	plt = get_ftrace_plt(mod);
> +	plt = get_ftrace_plt(mod, pc);
>  	if (!plt) {
>  		pr_err("ftrace: no module PLT for %ps\n", (void *)*addr);
>  		return false;
> diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
> index bde32979c06a..7afd370da9f4 100644
> --- a/arch/arm64/kernel/module-plts.c
> +++ b/arch/arm64/kernel/module-plts.c
> @@ -283,7 +283,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>  	unsigned long core_plts = 0;
>  	unsigned long init_plts = 0;
>  	Elf64_Sym *syms = NULL;
> -	Elf_Shdr *pltsec, *tramp = NULL;
> +	Elf_Shdr *pltsec, *tramp = NULL, *init_tramp = NULL;
>  	int i;
>  
>  	/*
> @@ -298,6 +298,9 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>  		else if (!strcmp(secstrings + sechdrs[i].sh_name,
>  				 ".text.ftrace_trampoline"))
>  			tramp = sechdrs + i;
> +		else if (!strcmp(secstrings + sechdrs[i].sh_name,
> +				 ".init.text.ftrace_trampoline"))
> +			init_tramp = sechdrs + i;
>  		else if (sechdrs[i].sh_type == SHT_SYMTAB)
>  			syms = (Elf64_Sym *)sechdrs[i].sh_addr;
>  	}
> @@ -363,5 +366,12 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>  		tramp->sh_size = NR_FTRACE_PLTS * sizeof(struct plt_entry);
>  	}
>  
> +	if (init_tramp) {
> +		init_tramp->sh_type = SHT_NOBITS;
> +		init_tramp->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
> +		init_tramp->sh_addralign = __alignof__(struct plt_entry);
> +		init_tramp->sh_size = NR_FTRACE_PLTS * sizeof(struct plt_entry);
> +	}
> +
>  	return 0;
>  }
> diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
> index 40148d2725ce..d6d443c4a01a 100644
> --- a/arch/arm64/kernel/module.c
> +++ b/arch/arm64/kernel/module.c
> @@ -466,6 +466,17 @@ static int module_init_ftrace_plt(const Elf_Ehdr *hdr,
>  	__init_plt(&plts[FTRACE_PLT_IDX], FTRACE_ADDR);
>  
>  	mod->arch.ftrace_trampolines = plts;
> +
> +	s = find_section(hdr, sechdrs, ".init.text.ftrace_trampoline");
> +	if (!s)
> +		return -ENOEXEC;
> +
> +	plts = (void *)s->sh_addr;
> +
> +	__init_plt(&plts[FTRACE_PLT_IDX], FTRACE_ADDR);
> +
> +	mod->arch.init_ftrace_trampolines = plts;
> +
>  #endif
>  	return 0;
>  }
> -- 
> 2.34.1
>