[PATCH] ARM: module: Teach unwinder about PLTs

[Alexander Sverdlin]

Hello Russel,

shall I add this one to your patch tracker?
There is little interest to this patch, but also no complaints, as I can tell.

On 06/07/2022 18:31, Alexander A Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin at nokia.com>
> 
> "unwind: Index not found eef26358" warnings keep popping up on
> CONFIG_ARM_MODULE_PLTS-enabled systems if the PC points to a PLT veneer.
> Teach the unwinder how to deal with them, taking into account they don't
> change state of the stack or register file except loading PC.
> 
> Tested-by: Kursad Oney <kursad.oney at broadcom.com>
> Link: https://lore.kernel.org/linux-arm-kernel/20200402153845.30985-1-kursad.oney@broadcom.com/
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin at nokia.com>
> ---
>  arch/arm/include/asm/module.h |  5 +++++
>  arch/arm/kernel/module-plts.c | 14 ++++++++++++++
>  arch/arm/kernel/unwind.c      | 13 ++++++++++++-
>  3 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/module.h b/arch/arm/include/asm/module.h
> index 5546c9751478c..07c51a34f77d5 100644
> --- a/arch/arm/include/asm/module.h
> +++ b/arch/arm/include/asm/module.h
> @@ -37,6 +37,11 @@ struct mod_arch_specific {
>  
>  struct module;
>  u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val);
> +#ifdef CONFIG_ARM_MODULE_PLTS
> +bool in_module_plt(unsigned long loc);
> +#else
> +static inline bool in_module_plt(unsigned long loc) { return false; }
> +#endif
>  
>  #ifdef CONFIG_THUMB2_KERNEL
>  #define HAVE_ARCH_KALLSYMS_SYMBOL_VALUE
> diff --git a/arch/arm/kernel/module-plts.c b/arch/arm/kernel/module-plts.c
> index 1fc309b41f944..af7c322ebed68 100644
> --- a/arch/arm/kernel/module-plts.c
> +++ b/arch/arm/kernel/module-plts.c
> @@ -284,3 +284,17 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>  		 mod->arch.core.plt->sh_size, mod->arch.init.plt->sh_size);
>  	return 0;
>  }
> +
> +bool in_module_plt(unsigned long loc)
> +{
> +	struct module *mod;
> +	bool ret;
> +
> +	preempt_disable();
> +	mod = __module_text_address(loc);
> +	ret = mod && (loc - (u32)mod->arch.core.plt_ent < mod->arch.core.plt_count * PLT_ENT_SIZE ||
> +		      loc - (u32)mod->arch.init.plt_ent < mod->arch.init.plt_count * PLT_ENT_SIZE);
> +	preempt_enable();
> +
> +	return ret;
> +}
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index a37ea6c772cd5..53be7ea6181b3 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -28,6 +28,7 @@
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
>  #include <linux/list.h>
> +#include <linux/module.h>
>  
>  #include <asm/stacktrace.h>
>  #include <asm/traps.h>
> @@ -395,8 +396,18 @@ int unwind_frame(struct stackframe *frame)
>  
>  	idx = unwind_find_idx(frame->pc);
>  	if (!idx) {
> -		if (frame->pc && kernel_text_address(frame->pc))
> +		if (frame->pc && kernel_text_address(frame->pc)) {
> +			if (in_module_plt(frame->pc) && frame->pc != frame->lr) {
> +				/*
> +				 * Quoting Ard: Veneers only set PC using a
> +				 * PC+immediate LDR, and so they don't affect
> +				 * the state of the stack or the register file
> +				 */
> +				frame->pc = frame->lr;
> +				return URC_OK;
> +			}
>  			pr_warn("unwind: Index not found %08lx\n", frame->pc);
> +		}
>  		return -URC_FAILURE;
>  	}
>  

-- 
Best regards,
Alexander Sverdlin.