Re: [PATCH v5 2/2] ARM: module: Fix function kallsyms on Thumb-2
From: Jessica Yu
Date: Thu Dec 06 2018 - 12:29:28 EST
+++ 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@xxxxxxxx>
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