ARM: module: Teach unwinder about PLTs

[PATCH] ARM: module: Teach unwinder about PLTs

Posted by Alexander A Sverdlin 3 years, 9 months ago

From: Alexander Sverdlin <alexander.sverdlin@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@broadcom.com>
Link: https://lore.kernel.org/linux-arm-kernel/20200402153845.30985-1-kursad.oney@broadcom.com/
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@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;
 	}
 
-- 
2.35.1

Re: [PATCH] ARM: module: Teach unwinder about PLTs

Posted by Linus Walleij 3 years, 7 months ago

On Wed, Jul 6, 2022 at 6:33 PM Alexander A Sverdlin
<alexander.sverdlin@nokia.com> wrote:

> From: Alexander Sverdlin <alexander.sverdlin@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@broadcom.com>
> Link: https://lore.kernel.org/linux-arm-kernel/20200402153845.30985-1-kursad.oney@broadcom.com/
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>

This looks correct to me.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Please add this to Russell's patch tracker.

Yours,
Linus Walleij

Re: [PATCH] ARM: module: Teach unwinder about PLTs

Posted by Alexander Sverdlin 3 years, 7 months ago

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@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@broadcom.com>
> Link: https://lore.kernel.org/linux-arm-kernel/20200402153845.30985-1-kursad.oney@broadcom.com/
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@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.

Re: [PATCH] ARM: module: Teach unwinder about PLTs

Posted by Florian Fainelli 3 years, 9 months ago

On 7/6/22 09:31, Alexander A Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin@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@broadcom.com>
> Link: https://lore.kernel.org/linux-arm-kernel/20200402153845.30985-1-kursad.oney@broadcom.com/
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>

I would be inclined to slap a Fixes tag to this patch so we get it back 
ported where appropriate, however it is not clear to me which specific
commit(s) we should be listing here.

Thanks Alexander!
-- 
Florian

Re: [PATCH] ARM: module: Teach unwinder about PLTs

Posted by Alexander Sverdlin 3 years, 9 months ago

Hello Florian,

On 06/07/2022 20:09, Florian Fainelli wrote:
>>
>> "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@broadcom.com>
>> Link: https://lore.kernel.org/linux-arm-kernel/20200402153845.30985-1-kursad.oney@broadcom.com/
>> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
> 
> I would be inclined to slap a Fixes tag to this patch so we get it back ported where appropriate, however it is not clear to me which specific
> commit(s) we should be listing here.

usually I don't hesitate to add "Fixes:" tag and this patch definitely could be ported to some
stable versions, but it's hard to say what it fixes. It's actually a combination of rarely
(if at all) used ARM_MODULE_PLTS functionality and other *debug* features, where unwinder is
used. In very similar case of commit 79f32b221b18c15a98507b101ef4beb52444cc6f
("ARM: 9079/1: ftrace: Add MODULE_PLTS support") I wasn't able to figure out which commit
I'm fixing.

I expect those who use ARM_MODULE_PLT and any kind of debugging to be able to backport the
patch downstream ;) 

-- 
Best regards,
Alexander Sverdlin.