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Subject[RFC 2/3] vsprintf: print <no-symbol> if symbol not found
Depends on: commit bd6b239cdbb2 ("kallsyms: don't leak address when
symbol not found")

Currently vsprintf for specifiers %p[SsB] relies on the behaviour of
kallsyms (sprint_symbol()) and prints the actual address if a symbol is
not found. Previous patch changes this behaviour so tha sprint_symbol()
returns an error if symbol not found. With this patch in place we can
print a sanitized message '<no-symbol>' instead of leaking the address.

Future users of vsprintf may wish to know, after a call that uses
specifier %p[sSB], whether or not a symbol was found. The actual
sanitized string should be contained (isolated) within the vsprintf.c
therefore we should provide a predicate function. This also allows the
sanitized string to be updated at a later stage with minimal risk to
calling code.

Print '<no-symbol>' for printk specifier %s[sSB] if no symbol is
found. Provide predicate function string_is_no_symbol().

Signed-off-by: Tobin C. Harding <>
include/linux/kernel.h | 2 ++
lib/vsprintf.c | 18 +++++++++++++++---
2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index ce51455e2adf..89e8ce79c2d1 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -460,6 +460,8 @@ char *kvasprintf(gfp_t gfp, const char *fmt, va_list args);
extern __printf(2, 0)
const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list args);

+extern int string_is_no_symbol(const char *s);
extern __scanf(2, 3)
int sscanf(const char *, const char *, ...);
extern __scanf(2, 0)
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 1746bae94d41..01e18a8c63fd 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -665,6 +665,8 @@ char *bdev_name(char *buf, char *end, struct block_device *bdev,

+#define PRINTK_NO_SYMBOL_STR "<no-symbol>"
static noinline_for_stack
char *symbol_string(char *buf, char *end, void *ptr,
struct printf_spec spec, const char *fmt)
@@ -672,6 +674,7 @@ char *symbol_string(char *buf, char *end, void *ptr,
unsigned long value;
char sym[KSYM_SYMBOL_LEN];
+ int ret;

if (fmt[1] == 'R')
@@ -680,11 +683,14 @@ char *symbol_string(char *buf, char *end, void *ptr,

if (*fmt == 'B')
- sprint_backtrace(sym, value);
+ ret = sprint_backtrace(sym, value);
else if (*fmt != 'f' && *fmt != 's')
- sprint_symbol(sym, value);
+ ret = sprint_symbol(sym, value);
- sprint_symbol_no_offset(sym, value);
+ ret = sprint_symbol_no_offset(sym, value);
+ if (ret == -1)
+ strcpy(sym, PRINTK_NO_SYMBOL_STR);

return string(buf, end, sym, spec);
@@ -692,6 +698,12 @@ char *symbol_string(char *buf, char *end, void *ptr,

+int string_is_no_symbol(const char *s)
+ return !!strstr(s, PRINTK_NO_SYMBOL_STR);
static noinline_for_stack
char *resource_string(char *buf, char *end, struct resource *res,
struct printf_spec spec, const char *fmt)
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