Re: [PATCH] kselftest: Minimise dependency of get_size on C library interfaces

[Masami Hiramatsu]

Hi,

[Cc: Tim Bird]

2020å1æ14æ(ç) 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