Hi, [Cc: Tim Bird] 2020年1月14日(火) 1:43 Siddhesh Poyarekar <siddhesh@xxxxxxxxxx>: > > It was observed[1] on arm64 that __builtin_strlen led to an infinite > loop in the get_size selftest. This is because __builtin_strlen (and > other builtins) may sometimes result in a call to the C library > function. The C library implementation of strlen uses an IFUNC > resolver to load the most efficient strlen implementation for the > underlying machine and hence has a PLT indirection even for static > binaries. Because this binary avoids the C library startup routines, > the PLT initialization never happens and hence the program gets stuck > in an infinite loop. > > On x86_64 the __builtin_strlen just happens to expand inline and avoid > the call but that is not always guaranteed. > > Further, while testing on x86_64 (Fedora 31), it was observed that the > test also failed with a segfault inside write() because the generated > code for the write function in glibc seems to access TLS before the > syscall (probably due to the cancellation point check) and fails > because TLS is not initialised. > > To mitigate these problems, this patch reduces the interface with the > C library to just the syscall function. The syscall function still > sets errno on failure, which is undesirable but for now it only > affects cases where syscalls fail. > > [1] https://bugs.linaro.org/show_bug.cgi?id=5479 > Thank you for the fix! I confirmed this fixes the issue. ---- root@devnote2:/opt/kselftest/size# ./get_size TAP version 13 # Testing system size. ok 1 get runtime memory use # System runtime memory report (units in Kilobytes): --- Total: 16085116 Free: 2042880 Buffer: 814052 In use: 13228184 ... 1..1 ---- Tested-by: Masami Hiramatsu <masami.hiramatsu@xxxxxxxxxx> > Signed-off-by: Siddhesh Poyarekar <siddhesh@xxxxxxxxxx> > Reported-by: Masami Hiramatsu <masami.hiramatsu@xxxxxxxxxx> > --- > tools/testing/selftests/size/get_size.c | 24 ++++++++++++++++++------ > 1 file changed, 18 insertions(+), 6 deletions(-) > > diff --git a/tools/testing/selftests/size/get_size.c b/tools/testing/selftests/size/get_size.c > index d4b59ab979a0..f55943b6d1e2 100644 > --- a/tools/testing/selftests/size/get_size.c > +++ b/tools/testing/selftests/size/get_size.c > @@ -12,23 +12,35 @@ > * own execution. It also attempts to have as few dependencies > * on kernel features as possible. > * > - * It should be statically linked, with startup libs avoided. > - * It uses no library calls, and only the following 3 syscalls: > + * It should be statically linked, with startup libs avoided. It uses > + * no library calls except the syscall() function for the following 3 > + * syscalls: > * sysinfo(), write(), and _exit() > * > * For output, it avoids printf (which in some C libraries > * has large external dependencies) by implementing it's own > * number output and print routines, and using __builtin_strlen() > + * > + * The test may crash if any of the above syscalls fails because in some > + * libc implementations (e.g. the GNU C Library) errno is saved in > + * thread-local storage, which does not get initialized due to avoiding > + * startup libs. > */ > > #include <sys/sysinfo.h> > #include <unistd.h> > +#include <sys/syscall.h> > > #define STDOUT_FILENO 1 > > static int print(const char *s) > { > - return write(STDOUT_FILENO, s, __builtin_strlen(s)); > + size_t len = 0; > + > + while (s[len] != '\0') > + len++; > + > + return syscall(SYS_write, STDOUT_FILENO, s, len); > } > > static inline char *num_to_str(unsigned long num, char *buf, int len) > @@ -80,12 +92,12 @@ void _start(void) > print("TAP version 13\n"); > print("# Testing system size.\n"); > > - ccode = sysinfo(&info); > + ccode = syscall(SYS_sysinfo, &info); > if (ccode < 0) { > print("not ok 1"); > print(test_name); > print(" ---\n reason: \"could not get sysinfo\"\n ...\n"); > - _exit(ccode); > + syscall(SYS_exit, ccode); > } > print("ok 1"); > print(test_name); > @@ -101,5 +113,5 @@ void _start(void) > print(" ...\n"); > print("1..1\n"); > > - _exit(0); > + syscall(SYS_exit, 0); > } > -- > 2.24.1 > -- Masami Hiramatsu
