[PATCH v5 1/2] module: Overwrite st_size instead of st_info

[PATCH v5 1/2] module: Overwrite st_size instead of st_info

[Vincent Whitchurch]

st_info is currently overwritten after relocation and used to store the
elf_type().  However, we're going to need it fix kallsyms on ARM's
Thumb-2 kernels, so preserve st_info and overwrite the st_size field
instead.  st_size is neither used by the module core nor by any
architecture.

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
v5: Add Dave Martin's Reviewed-by
v4: Split out to separate patch.  Use st_size instead of st_other.
v1-v3: See PATCH 2/2

 kernel/module.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 49a405891587..3d86a38b580c 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2682,7 +2682,7 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
 
 	/* Set types up while we still have access to sections. */
 	for (i = 0; i < mod->kallsyms->num_symtab; i++)
-		mod->kallsyms->symtab[i].st_info
+		mod->kallsyms->symtab[i].st_size
 			= elf_type(&mod->kallsyms->symtab[i], info);
 
 	/* Now populate the cut down core kallsyms for after init. */
@@ -4061,7 +4061,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
 		kallsyms = rcu_dereference_sched(mod->kallsyms);
 		if (symnum < kallsyms->num_symtab) {
 			*value = kallsyms->symtab[symnum].st_value;
-			*type = kallsyms->symtab[symnum].st_info;
+			*type = kallsyms->symtab[symnum].st_size;
 			strlcpy(name, symname(kallsyms, symnum), KSYM_NAME_LEN);
 			strlcpy(module_name, mod->name, MODULE_NAME_LEN);
 			*exported = is_exported(name, *value, mod);
-- 
2.11.0

[PATCH v5 2/2] ARM: module: Fix function kallsyms on Thumb-2

[Vincent Whitchurch]

Thumb-2 functions have the lowest bit set in the symbol value in the
symtab.  When kallsyms are generated for the vmlinux, the kallsyms are
generated from the output of nm, and nm clears the lowest bit.

 $ arm-linux-gnueabihf-readelf -a vmlinux | grep show_interrupts
  95947: 8015dc89   686 FUNC    GLOBAL DEFAULT    2 show_interrupts
 $ arm-linux-gnueabihf-nm vmlinux | grep show_interrupts
 8015dc88 T show_interrupts
 $ cat /proc/kallsyms | grep show_interrupts
 8015dc88 T show_interrupts

However, for modules, the kallsyms uses the values in the symbol table
without modification, so for functions in modules, the lowest bit is set
in kallsyms.

 $ arm-linux-gnueabihf-readelf -a drivers/net/tun.ko | grep tun_get_socket
    333: 00002d4d    36 FUNC    GLOBAL DEFAULT    1 tun_get_socket
 $ arm-linux-gnueabihf-nm drivers/net/tun.ko | grep tun_get_socket
 00002d4c T tun_get_socket
 $ cat /proc/kallsyms | grep tun_get_socket
 7f802d4d t tun_get_socket      [tun]

Because of this, the symbol+offset of the crashing instruction shown in
oopses is incorrect when the crash is in a module.  For example, given a
tun_get_socket which starts like this,

 00002d4c <tun_get_socket>:
     2d4c:       6943            ldr     r3, [r0, #20]
     2d4e:       4a07            ldr     r2, [pc, #28]
     2d50:       4293            cmp     r3, r2

a crash when tun_get_socket is called with NULL results in:

 PC is at tun_xdp+0xa3/0xa4 [tun]
 pc : [<7f802d4c>]

As can be seen, the "PC is at" line reports the wrong symbol name, and
the symbol+offset will point to the wrong source line if it is passed to
gdb.

To solve this, add a way for archs to fixup the reading of these module
kallsyms values, and use that to clear the lowest bit for function
symbols on Thumb-2.

After the fix:

 # cat /proc/kallsyms | grep tun_get_socket
 7f802d4c t tun_get_socket       [tun]

 PC is at tun_get_socket+0x0/0x24 [tun]
 pc : [<7f802d4c>]

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
v5: Use/move local variables to reduce calls and keep lines short.  Use const arg.
v4: Split out st_value overwrite change.  Add HAVE* macro to avoid function call.
v3: Do not overwrite st_value
v2: Fix build warning with !MODULES

 arch/arm/include/asm/module.h | 11 +++++++++++
 include/linux/module.h        |  7 +++++++
 kernel/module.c               | 45 +++++++++++++++++++++++++++----------------
 3 files changed, 46 insertions(+), 17 deletions(-)

diff --git a/arch/arm/include/asm/module.h b/arch/arm/include/asm/module.h
index 9e81b7c498d8..c7bcf0347801 100644
--- a/arch/arm/include/asm/module.h
+++ b/arch/arm/include/asm/module.h
@@ -61,4 +61,15 @@ u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val);
 	MODULE_ARCH_VERMAGIC_ARMTHUMB \
 	MODULE_ARCH_VERMAGIC_P2V
 
+#ifdef CONFIG_THUMB2_KERNEL
+#define HAVE_ARCH_MODULE_KALLSYMS_SYMBOL_VALUE
+static inline unsigned long module_kallsyms_symbol_value(const Elf_Sym *sym)
+{
+	if (ELF_ST_TYPE(sym->st_info) == STT_FUNC)
+		return sym->st_value & ~1;
+
+	return sym->st_value;
+}
+#endif
+
 #endif /* _ASM_ARM_MODULE_H */
diff --git a/include/linux/module.h b/include/linux/module.h
index fce6b4335e36..12146257eb5d 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -486,6 +486,13 @@ struct module {
 #define MODULE_ARCH_INIT {}
 #endif
 
+#ifndef HAVE_ARCH_MODULE_KALLSYMS_SYMBOL_VALUE
+static inline unsigned long module_kallsyms_symbol_value(const Elf_Sym *sym)
+{
+	return sym->st_value;
+}
+#endif
+
 extern struct mutex module_mutex;
 
 /* FIXME: It'd be nice to isolate modules during init, too, so they
diff --git a/kernel/module.c b/kernel/module.c
index 3d86a38b580c..9364017fdc21 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3922,7 +3922,7 @@ static const char *get_ksymbol(struct module *mod,
 			       unsigned long *offset)
 {
 	unsigned int i, best = 0;
-	unsigned long nextval;
+	unsigned long nextval, bestval;
 	struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
 
 	/* At worse, next value is at end of module */
@@ -3931,10 +3931,15 @@ static const char *get_ksymbol(struct module *mod,
 	else
 		nextval = (unsigned long)mod->core_layout.base+mod->core_layout.text_size;
 
+	bestval = module_kallsyms_symbol_value(&kallsyms->symtab[best]);
+
 	/* Scan for closest preceding symbol, and next symbol. (ELF
 	   starts real symbols at 1). */
 	for (i = 1; i < kallsyms->num_symtab; i++) {
-		if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
+		const Elf_Sym *sym = &kallsyms->symtab[i];
+		unsigned long thisval = module_kallsyms_symbol_value(sym);
+
+		if (sym->st_shndx == SHN_UNDEF)
 			continue;
 
 		/* We ignore unnamed symbols: they're uninformative
@@ -3943,21 +3948,21 @@ static const char *get_ksymbol(struct module *mod,
 		    || is_arm_mapping_symbol(symname(kallsyms, i)))
 			continue;
 
-		if (kallsyms->symtab[i].st_value <= addr
-		    && kallsyms->symtab[i].st_value > kallsyms->symtab[best].st_value)
+		if (thisval <= addr && thisval > bestval) {
 			best = i;
-		if (kallsyms->symtab[i].st_value > addr
-		    && kallsyms->symtab[i].st_value < nextval)
-			nextval = kallsyms->symtab[i].st_value;
+			bestval = thisval;
+		}
+		if (thisval > addr && thisval < nextval)
+			nextval = thisval;
 	}
 
 	if (!best)
 		return NULL;
 
 	if (size)
-		*size = nextval - kallsyms->symtab[best].st_value;
+		*size = nextval - bestval;
 	if (offset)
-		*offset = addr - kallsyms->symtab[best].st_value;
+		*offset = addr - bestval;
 	return symname(kallsyms, best);
 }
 
@@ -4060,8 +4065,10 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
 			continue;
 		kallsyms = rcu_dereference_sched(mod->kallsyms);
 		if (symnum < kallsyms->num_symtab) {
-			*value = kallsyms->symtab[symnum].st_value;
-			*type = kallsyms->symtab[symnum].st_size;
+			const Elf_Sym *sym = &kallsyms->symtab[symnum];
+
+			*value = module_kallsyms_symbol_value(sym);
+			*type = sym->st_size;
 			strlcpy(name, symname(kallsyms, symnum), KSYM_NAME_LEN);
 			strlcpy(module_name, mod->name, MODULE_NAME_LEN);
 			*exported = is_exported(name, *value, mod);
@@ -4079,10 +4086,13 @@ static unsigned long mod_find_symname(struct module *mod, const char *name)
 	unsigned int i;
 	struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
 
-	for (i = 0; i < kallsyms->num_symtab; i++)
+	for (i = 0; i < kallsyms->num_symtab; i++) {
+		const Elf_Sym *sym = &kallsyms->symtab[i];
+
 		if (strcmp(name, symname(kallsyms, i)) == 0 &&
-		    kallsyms->symtab[i].st_shndx != SHN_UNDEF)
-			return kallsyms->symtab[i].st_value;
+		    sym->st_shndx != SHN_UNDEF)
+			return module_kallsyms_symbol_value(sym);
+	}
 	return 0;
 }
 
@@ -4127,12 +4137,13 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
 		if (mod->state == MODULE_STATE_UNFORMED)
 			continue;
 		for (i = 0; i < kallsyms->num_symtab; i++) {
+			const Elf_Sym *sym = &kallsyms->symtab[i];
 
-			if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
+			if (sym->st_shndx == SHN_UNDEF)
 				continue;
 
-			ret = fn(data, symname(kallsyms, i),
-				 mod, kallsyms->symtab[i].st_value);
+			ret = fn(data, symname(kallsyms, i), mod,
+				 module_kallsyms_symbol_value(sym));
 			if (ret != 0)
 				return ret;
 		}
-- 
2.11.0

Re: [PATCH v5 1/2] module: Overwrite st_size instead of st_info

[Miroslav Benes]

On Tue, 4 Dec 2018, Vincent Whitchurch wrote:

> st_info is currently overwritten after relocation and used to store the
> elf_type().  However, we're going to need it fix kallsyms on ARM's
> Thumb-2 kernels, so preserve st_info and overwrite the st_size field
> instead.  st_size is neither used by the module core nor by any
> architecture.
> 
> Reviewed-by: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>

Reviewed-by: Miroslav Benes <mbenes@suse.cz>

M

Re: [PATCH v5 2/2] ARM: module: Fix function kallsyms on Thumb-2

[Jessica Yu]

+++ Vincent Whitchurch [04/12/18 15:14 +0100]:
>Thumb-2 functions have the lowest bit set in the symbol value in the
>symtab.  When kallsyms are generated for the vmlinux, the kallsyms are
>generated from the output of nm, and nm clears the lowest bit.
>
> $ arm-linux-gnueabihf-readelf -a vmlinux | grep show_interrupts
>  95947: 8015dc89   686 FUNC    GLOBAL DEFAULT    2 show_interrupts
> $ arm-linux-gnueabihf-nm vmlinux | grep show_interrupts
> 8015dc88 T show_interrupts
> $ cat /proc/kallsyms | grep show_interrupts
> 8015dc88 T show_interrupts
>
>However, for modules, the kallsyms uses the values in the symbol table
>without modification, so for functions in modules, the lowest bit is set
>in kallsyms.
>
> $ arm-linux-gnueabihf-readelf -a drivers/net/tun.ko | grep tun_get_socket
>    333: 00002d4d    36 FUNC    GLOBAL DEFAULT    1 tun_get_socket
> $ arm-linux-gnueabihf-nm drivers/net/tun.ko | grep tun_get_socket
> 00002d4c T tun_get_socket
> $ cat /proc/kallsyms | grep tun_get_socket
> 7f802d4d t tun_get_socket      [tun]
>
>Because of this, the symbol+offset of the crashing instruction shown in
>oopses is incorrect when the crash is in a module.  For example, given a
>tun_get_socket which starts like this,
>
> 00002d4c <tun_get_socket>:
>     2d4c:       6943            ldr     r3, [r0, #20]
>     2d4e:       4a07            ldr     r2, [pc, #28]
>     2d50:       4293            cmp     r3, r2
>
>a crash when tun_get_socket is called with NULL results in:
>
> PC is at tun_xdp+0xa3/0xa4 [tun]
> pc : [<7f802d4c>]
>
>As can be seen, the "PC is at" line reports the wrong symbol name, and
>the symbol+offset will point to the wrong source line if it is passed to
>gdb.
>
>To solve this, add a way for archs to fixup the reading of these module
>kallsyms values, and use that to clear the lowest bit for function
>symbols on Thumb-2.
>
>After the fix:
>
> # cat /proc/kallsyms | grep tun_get_socket
> 7f802d4c t tun_get_socket       [tun]
>
> PC is at tun_get_socket+0x0/0x24 [tun]
> pc : [<7f802d4c>]
>
>Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>

Looks good, could I get an ACK from an ARM maintainer? (Russell?)

Also, do you mind if I drop the module_ prefix from
module_kallsyms_symbol_value()? I recently submitted some internal
module kallsyms cleanups [1] and there we have the newly renamed
kallsyms_symbol_name(), so I think it'd be nice if we kept the naming
scheme consistent with just kallsyms_symbol_value(). I could just do
this myself if you don't want to respin a v6.

Thanks!

Jessica

[1] https://git.kernel.org/pub/scm/linux/kernel/git/jeyu/linux.git/commit/?h=modules-next&id=2d25bc55235314d869459c574be14e8faa73aca3

>---
>v5: Use/move local variables to reduce calls and keep lines short.  Use const arg.
>v4: Split out st_value overwrite change.  Add HAVE* macro to avoid function call.
>v3: Do not overwrite st_value
>v2: Fix build warning with !MODULES
>
> arch/arm/include/asm/module.h | 11 +++++++++++
> include/linux/module.h        |  7 +++++++
> kernel/module.c               | 45 +++++++++++++++++++++++++++----------------
> 3 files changed, 46 insertions(+), 17 deletions(-)
>
>diff --git a/arch/arm/include/asm/module.h b/arch/arm/include/asm/module.h
>index 9e81b7c498d8..c7bcf0347801 100644
>--- a/arch/arm/include/asm/module.h
>+++ b/arch/arm/include/asm/module.h
>@@ -61,4 +61,15 @@ u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val);
> 	MODULE_ARCH_VERMAGIC_ARMTHUMB \
> 	MODULE_ARCH_VERMAGIC_P2V
>
>+#ifdef CONFIG_THUMB2_KERNEL
>+#define HAVE_ARCH_MODULE_KALLSYMS_SYMBOL_VALUE
>+static inline unsigned long module_kallsyms_symbol_value(const Elf_Sym *sym)
>+{
>+	if (ELF_ST_TYPE(sym->st_info) == STT_FUNC)
>+		return sym->st_value & ~1;
>+
>+	return sym->st_value;
>+}
>+#endif
>+
> #endif /* _ASM_ARM_MODULE_H */
>diff --git a/include/linux/module.h b/include/linux/module.h
>index fce6b4335e36..12146257eb5d 100644
>--- a/include/linux/module.h
>+++ b/include/linux/module.h
>@@ -486,6 +486,13 @@ struct module {
> #define MODULE_ARCH_INIT {}
> #endif
>
>+#ifndef HAVE_ARCH_MODULE_KALLSYMS_SYMBOL_VALUE
>+static inline unsigned long module_kallsyms_symbol_value(const Elf_Sym *sym)
>+{
>+	return sym->st_value;
>+}
>+#endif
>+
> extern struct mutex module_mutex;
>
> /* FIXME: It'd be nice to isolate modules during init, too, so they
>diff --git a/kernel/module.c b/kernel/module.c
>index 3d86a38b580c..9364017fdc21 100644
>--- a/kernel/module.c
>+++ b/kernel/module.c
>@@ -3922,7 +3922,7 @@ static const char *get_ksymbol(struct module *mod,
> 			       unsigned long *offset)
> {
> 	unsigned int i, best = 0;
>-	unsigned long nextval;
>+	unsigned long nextval, bestval;
> 	struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
>
> 	/* At worse, next value is at end of module */
>@@ -3931,10 +3931,15 @@ static const char *get_ksymbol(struct module *mod,
> 	else
> 		nextval = (unsigned long)mod->core_layout.base+mod->core_layout.text_size;
>
>+	bestval = module_kallsyms_symbol_value(&kallsyms->symtab[best]);
>+
> 	/* Scan for closest preceding symbol, and next symbol. (ELF
> 	   starts real symbols at 1). */
> 	for (i = 1; i < kallsyms->num_symtab; i++) {
>-		if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
>+		const Elf_Sym *sym = &kallsyms->symtab[i];
>+		unsigned long thisval = module_kallsyms_symbol_value(sym);
>+
>+		if (sym->st_shndx == SHN_UNDEF)
> 			continue;
>
> 		/* We ignore unnamed symbols: they're uninformative
>@@ -3943,21 +3948,21 @@ static const char *get_ksymbol(struct module *mod,
> 		    || is_arm_mapping_symbol(symname(kallsyms, i)))
> 			continue;
>
>-		if (kallsyms->symtab[i].st_value <= addr
>-		    && kallsyms->symtab[i].st_value > kallsyms->symtab[best].st_value)
>+		if (thisval <= addr && thisval > bestval) {
> 			best = i;
>-		if (kallsyms->symtab[i].st_value > addr
>-		    && kallsyms->symtab[i].st_value < nextval)
>-			nextval = kallsyms->symtab[i].st_value;
>+			bestval = thisval;
>+		}
>+		if (thisval > addr && thisval < nextval)
>+			nextval = thisval;
> 	}
>
> 	if (!best)
> 		return NULL;
>
> 	if (size)
>-		*size = nextval - kallsyms->symtab[best].st_value;
>+		*size = nextval - bestval;
> 	if (offset)
>-		*offset = addr - kallsyms->symtab[best].st_value;
>+		*offset = addr - bestval;
> 	return symname(kallsyms, best);
> }
>
>@@ -4060,8 +4065,10 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
> 			continue;
> 		kallsyms = rcu_dereference_sched(mod->kallsyms);
> 		if (symnum < kallsyms->num_symtab) {
>-			*value = kallsyms->symtab[symnum].st_value;
>-			*type = kallsyms->symtab[symnum].st_size;
>+			const Elf_Sym *sym = &kallsyms->symtab[symnum];
>+
>+			*value = module_kallsyms_symbol_value(sym);
>+			*type = sym->st_size;
> 			strlcpy(name, symname(kallsyms, symnum), KSYM_NAME_LEN);
> 			strlcpy(module_name, mod->name, MODULE_NAME_LEN);
> 			*exported = is_exported(name, *value, mod);
>@@ -4079,10 +4086,13 @@ static unsigned long mod_find_symname(struct module *mod, const char *name)
> 	unsigned int i;
> 	struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
>
>-	for (i = 0; i < kallsyms->num_symtab; i++)
>+	for (i = 0; i < kallsyms->num_symtab; i++) {
>+		const Elf_Sym *sym = &kallsyms->symtab[i];
>+
> 		if (strcmp(name, symname(kallsyms, i)) == 0 &&
>-		    kallsyms->symtab[i].st_shndx != SHN_UNDEF)
>-			return kallsyms->symtab[i].st_value;
>+		    sym->st_shndx != SHN_UNDEF)
>+			return module_kallsyms_symbol_value(sym);
>+	}
> 	return 0;
> }
>
>@@ -4127,12 +4137,13 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
> 		if (mod->state == MODULE_STATE_UNFORMED)
> 			continue;
> 		for (i = 0; i < kallsyms->num_symtab; i++) {
>+			const Elf_Sym *sym = &kallsyms->symtab[i];
>
>-			if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
>+			if (sym->st_shndx == SHN_UNDEF)
> 				continue;
>
>-			ret = fn(data, symname(kallsyms, i),
>-				 mod, kallsyms->symtab[i].st_value);
>+			ret = fn(data, symname(kallsyms, i), mod,
>+				 module_kallsyms_symbol_value(sym));
> 			if (ret != 0)
> 				return ret;
> 		}
>-- 
>2.11.0
>

Re: [PATCH v5 2/2] ARM: module: Fix function kallsyms on Thumb-2

[Vincent Whitchurch]

On Thu, Dec 06, 2018 at 06:29:20PM +0100, Jessica Yu wrote:
> Also, do you mind if I drop the module_ prefix from
> module_kallsyms_symbol_value()? I recently submitted some internal
> module kallsyms cleanups [1] and there we have the newly renamed
> kallsyms_symbol_name(), so I think it'd be nice if we kept the naming
> scheme consistent with just kallsyms_symbol_value(). I could just do
> this myself if you don't want to respin a v6.

I've sent out a v6 with this change.  I also rebased the change on top
of your cleanups in modules-next.

Thanks.