When built at -Os, gcc-12 recognizes an strlen() pattern in nolibc_strlen() and replaces it with a jump to strlen(), which is not defined as a symbol and breaks compilation. Worse, when the function is called strlen(), the function is simply replaced with a jump to itself, hence becomes an infinite loop.
One way to avoid this is to always set -ffreestanding, but the calling code doesn't know this and there's no way (either via attributes or pragmas) to globally enable it from include files, effectively leaving a painful situation for the caller.
It turns out that -fno-tree-loop-distribute-patterns disables replacement of strlen-like loops with calls to strlen and that this option is accepted in the optimize() function attribute. Thus at least it allows us to make sure our local definition is not replaced with a self jump. The function only needs to be renamed back to strlen() so that the symbol exists, which implies that nolibc_strlen() which is used on variable strings has to be declared as a macro that points back to it before the strlen() macro is redifined.
It was verified to produce valid code with gcc 3.4 to 12.1 at different optimization levels, and both with constant and variable strings.
Reported-by: kernel test robot yujie.liu@intel.com Link: https://lore.kernel.org/r/202210081618.754a77db-yujie.liu@intel.com Fixes: 66b6f755ad45 ("rcutorture: Import a copy of nolibc") Fixes: 96980b833a21 ("tools/nolibc/string: do not use __builtin_strlen() at -O0") Cc: "Paul E. McKenney" paulmck@kernel.org Signed-off-by: Willy Tarreau w@1wt.eu --- tools/include/nolibc/string.h | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h index bef35bee9c44..5ef8778cd16f 100644 --- a/tools/include/nolibc/string.h +++ b/tools/include/nolibc/string.h @@ -125,10 +125,16 @@ char *strcpy(char *dst, const char *src) }
/* this function is only used with arguments that are not constants or when - * it's not known because optimizations are disabled. + * it's not known because optimizations are disabled. Note that gcc 12 + * recognizes an strlen() pattern and replaces it with a jump to strlen(), + * thus itself, hence the optimize() attribute below that's meant to disable + * this confusing practice. */ +#if defined(__GNUC__) && (__GNUC__ >= 12) +__attribute__((optimize("no-tree-loop-distribute-patterns"))) +#endif static __attribute__((unused)) -size_t nolibc_strlen(const char *str) +size_t strlen(const char *str) { size_t len;
@@ -140,13 +146,12 @@ size_t nolibc_strlen(const char *str) * the two branches, then will rely on an external definition of strlen(). */ #if defined(__OPTIMIZE__) +#define nolibc_strlen(x) strlen(x) #define strlen(str) ({ \ __builtin_constant_p((str)) ? \ __builtin_strlen((str)) : \ nolibc_strlen((str)); \ }) -#else -#define strlen(str) nolibc_strlen((str)) #endif
static __attribute__((unused))
When built at -Os, gcc-12 recognizes an strlen() pattern in nolibc_strlen() and replaces it with a jump to strlen(), which is not defined as a symbol and breaks compilation. Worse, when the function is called strlen(), the function is simply replaced with a jump to itself, hence becomes an infinite loop.
One way to avoid this is to always set -ffreestanding, but the calling code doesn't know this and there's no way (either via attributes or pragmas) to globally enable it from include files, effectively leaving a painful situation for the caller.
Alexey suggested to place an empty asm() statement inside the loop to stop gcc from recognizing a well-known pattern, which happens to work pretty fine. At least it allows us to make sure our local definition is not replaced with a self jump.
The function only needs to be renamed back to strlen() so that the symbol exists, which implies that nolibc_strlen() which is used on variable strings has to be declared as a macro that points back to it before the strlen() macro is redifined.
It was verified to produce valid code with gcc 3.4 to 12.1 at different optimization levels, and both with constant and variable strings.
In case this problem surfaces again in the future, an alternate approach consisting in adding an optimize("no-tree-loop-distribute-patterns") function attribute for gcc>=12 worked as well but is less pretty.
Reported-by: kernel test robot yujie.liu@intel.com Link: https://lore.kernel.org/r/202210081618.754a77db-yujie.liu@intel.com Fixes: 66b6f755ad45 ("rcutorture: Import a copy of nolibc") Fixes: 96980b833a21 ("tools/nolibc/string: do not use __builtin_strlen() at -O0") Cc: "Paul E. McKenney" paulmck@kernel.org Cc: Alexey Dobriyan adobriyan@gmail.com Signed-off-by: Willy Tarreau w@1wt.eu --- v2: dropped the attribute(optimize) in favor of an empty asm() statement
--- tools/include/nolibc/string.h | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h index bef35bee9c44..718a405ffbc3 100644 --- a/tools/include/nolibc/string.h +++ b/tools/include/nolibc/string.h @@ -125,14 +125,18 @@ char *strcpy(char *dst, const char *src) }
/* this function is only used with arguments that are not constants or when - * it's not known because optimizations are disabled. + * it's not known because optimizations are disabled. Note that gcc 12 + * recognizes an strlen() pattern and replaces it with a jump to strlen(), + * thus itself, hence the asm() statement below that's meant to disable this + * confusing practice. */ static __attribute__((unused)) -size_t nolibc_strlen(const char *str) +size_t strlen(const char *str) { size_t len;
- for (len = 0; str[len]; len++); + for (len = 0; str[len]; len++) + asm(""); return len; }
@@ -140,13 +144,12 @@ size_t nolibc_strlen(const char *str) * the two branches, then will rely on an external definition of strlen(). */ #if defined(__OPTIMIZE__) +#define nolibc_strlen(x) strlen(x) #define strlen(str) ({ \ __builtin_constant_p((str)) ? \ __builtin_strlen((str)) : \ nolibc_strlen((str)); \ }) -#else -#define strlen(str) nolibc_strlen((str)) #endif
static __attribute__((unused))
On Sun, Oct 09, 2022 at 08:29:36PM +0200, Willy Tarreau wrote:
When built at -Os, gcc-12 recognizes an strlen() pattern in nolibc_strlen() and replaces it with a jump to strlen(), which is not defined as a symbol and breaks compilation. Worse, when the function is called strlen(), the function is simply replaced with a jump to itself, hence becomes an infinite loop.
One way to avoid this is to always set -ffreestanding, but the calling code doesn't know this and there's no way (either via attributes or pragmas) to globally enable it from include files, effectively leaving a painful situation for the caller.
Alexey suggested to place an empty asm() statement inside the loop to stop gcc from recognizing a well-known pattern, which happens to work pretty fine. At least it allows us to make sure our local definition is not replaced with a self jump.
The function only needs to be renamed back to strlen() so that the symbol exists, which implies that nolibc_strlen() which is used on variable strings has to be declared as a macro that points back to it before the strlen() macro is redifined.
It was verified to produce valid code with gcc 3.4 to 12.1 at different optimization levels, and both with constant and variable strings.
In case this problem surfaces again in the future, an alternate approach consisting in adding an optimize("no-tree-loop-distribute-patterns") function attribute for gcc>=12 worked as well but is less pretty.
Reported-by: kernel test robot yujie.liu@intel.com Link: https://lore.kernel.org/r/202210081618.754a77db-yujie.liu@intel.com Fixes: 66b6f755ad45 ("rcutorture: Import a copy of nolibc") Fixes: 96980b833a21 ("tools/nolibc/string: do not use __builtin_strlen() at -O0") Cc: "Paul E. McKenney" paulmck@kernel.org Cc: Alexey Dobriyan adobriyan@gmail.com Signed-off-by: Willy Tarreau w@1wt.eu
Hearing no objections, I have pulled this one in for review and testing, thank you!
Thanx, Paul
v2: dropped the attribute(optimize) in favor of an empty asm() statement
tools/include/nolibc/string.h | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h index bef35bee9c44..718a405ffbc3 100644 --- a/tools/include/nolibc/string.h +++ b/tools/include/nolibc/string.h @@ -125,14 +125,18 @@ char *strcpy(char *dst, const char *src) } /* this function is only used with arguments that are not constants or when
- it's not known because optimizations are disabled.
- it's not known because optimizations are disabled. Note that gcc 12
- recognizes an strlen() pattern and replaces it with a jump to strlen(),
- thus itself, hence the asm() statement below that's meant to disable this
*/
- confusing practice.
static __attribute__((unused)) -size_t nolibc_strlen(const char *str) +size_t strlen(const char *str) { size_t len;
- for (len = 0; str[len]; len++);
- for (len = 0; str[len]; len++)
asm("");} @@ -140,13 +144,12 @@ size_t nolibc_strlen(const char *str)
- the two branches, then will rely on an external definition of strlen().
*/ #if defined(__OPTIMIZE__) +#define nolibc_strlen(x) strlen(x) #define strlen(str) ({ \ __builtin_constant_p((str)) ? \ __builtin_strlen((str)) : \ nolibc_strlen((str)); \ }) -#else -#define strlen(str) nolibc_strlen((str)) #endif static __attribute__((unused)) -- 2.35.3
linux-kselftest-mirror@lists.linaro.org
-
Paul E. McKenney -
Willy Tarreau