Re: [PATCH v2 09/10] Warn when gettimeofday is called with non-null tzp argument.

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 28/08/2019 12:32, Zack Weinberg wrote:
> Since there are no known uses of gettimeofday's vestigial "get time
> zone" feature that are not bugs, add a fortify-style wrapper inline to
> sys/time.h that issues a warning whenever gettimeofday is called with
> a second argument that is not a compile-time null pointer
> constant.
> 
> At present this is only possible with GCC; clang does not implement
> attribute((warning)).  The wrapper is only activated when __OPTIMIZE__
> is defined because it throws false positives when optimization is off,
> even though it's an always-inline function.
> 
> An oversight in the implementation of __builtin_constant_p causes it
> to fail to detect compile-time *pointer* constants unless they are
> cast to an integer of a different size.  (Loss of data in this cast is
> harmless; the overall expression is still constant if and only if the
> original pointer was.)  This is GCC bug 95514.  Thanks to
> Kamil Cukrowski <kamilcukrowski@gmail.com> for the workaround.
> As a precaution, I added a static assertion to debug/warning-nop.c to
> make sure that the cast _is_ casting to an integer of a different
> size; this is too unlikely a scenario to be worth checking in the
> public header, but if someone ever adds a port where short is the
> same size as intptr_t, we'll still catch it.

The conditionals of making this work without false-positive seems quite 
specific and fragile (gcc-only, optimized build) and the cast hack make
even more suspicious it won't break in some arcane build environment 
(since it is an exported header, not internally used).

I would prefer to just wait gcc to correctly fix it and set its minimum 
version to enable it with the expected semantic.

> 
> Also make the public prototype of gettimeofday declare its second
> argument with type "void *" unconditionally, consistent with POSIX.

This is ok though.

> 
> 	* time/sys/time.h (__timezone_ptr_t): Delete.
> 	(gettimeofday): Always declare second argument with type "void *".
> 	When possible, wrap with a fortify-style inline function that
> 	detects non-null or non-constant second argument and issues a
> 	warning.  Improve commentary.
> 	(settimeofday): Improve commentary.
> 
> 	* time/gettimeofday.c (gettimeofday): Declare second argument with
> 	type "void *".
> 	* debug/warning-nop.c: Include sys/time.h and stdint.h.
> 	Add static_assert to verify the requirements of the workaround
> 	for GCC bug 95514.
> ---
>  debug/warning-nop.c |  9 +++++++++
>  time/gettimeofday.c |  4 ++--
>  time/sys/time.h     | 47 ++++++++++++++++++++++++++++++++++-----------
>  3 files changed, 47 insertions(+), 13 deletions(-)
> 
> diff --git a/debug/warning-nop.c b/debug/warning-nop.c
> index 8eeea396c3..3eab53b78f 100644
> --- a/debug/warning-nop.c
> +++ b/debug/warning-nop.c
> @@ -67,4 +67,13 @@ nop (void)
>  #define __builtin___strncpy_chk(dest, src, len, bos) NULL
>  #define __builtin_object_size(bos, level) 0
>  
> +/* The code in sys/time.h that uses __warndecl has to work around GCC
> +    bug 91554.  The work-around is only effective if intptr_t is not
> +    the same size as short.  */
> +#include <stdint.h>
> +_Static_assert (sizeof (intptr_t) != sizeof (short),
> +                "workaround for GCC bug 91554 in sys/time.h"
> +                " is only effective when short is smaller than a pointer");
> +
>  #include <string.h>
> +#include <sys/time.h>
> diff --git a/time/gettimeofday.c b/time/gettimeofday.c
> index c4f642631f..5bc91fc214 100644
> --- a/time/gettimeofday.c
> +++ b/time/gettimeofday.c
> @@ -23,10 +23,10 @@
>     If *TZ is not NULL, clear it.
>     Returns 0 on success, -1 on errors.  */
>  int
> -___gettimeofday (struct timeval *tv, struct timezone *tz)
> +___gettimeofday (struct timeval *restrict tv, void *restrict tz)
>  {
>    if (__glibc_unlikely (tz != 0))
> -    memset (tz, 0, sizeof *tz);
> +    memset (tz, 0, sizeof (struct timezone));
>  
>    struct timespec ts;
>    if (__clock_gettime (CLOCK_REALTIME, &ts))
> diff --git a/time/sys/time.h b/time/sys/time.h
> index 5dbc7fc627..a4e7fd20d1 100644
> --- a/time/sys/time.h
> +++ b/time/sys/time.h
> @@ -54,23 +54,48 @@ struct timezone
>      int tz_minuteswest;		/* Minutes west of GMT.  */
>      int tz_dsttime;		/* Nonzero if DST is ever in effect.  */
>    };
> -
> -typedef struct timezone *__restrict __timezone_ptr_t;
> -#else
> -typedef void *__restrict __timezone_ptr_t;
>  #endif
>  
> -/* Get the current time of day and timezone information,
> -   putting it into *TV and *TZ.  If TZ is NULL, *TZ is not filled.
> -   Returns 0 on success, -1 on errors.
> -   NOTE: This form of timezone information is obsolete.
> -   Use the functions and variables declared in <time.h> instead.  */
> +/* Get the current time of day, putting it into *TV.
> +   If TZ is not null, *TZ must be a struct timezone, and both fields
> +   will be set to zero.
> +   Calling this function with a non-null TZ is obsolete;
> +   use localtime etc. instead.
> +   This function itself is semi-obsolete;
> +   most callers should use time or clock_gettime instead. */
>  extern int gettimeofday (struct timeval *__restrict __tv,
> -			 __timezone_ptr_t __tz) __THROW __nonnull ((1));
> +			 void *__restrict __tz) __THROW __nonnull ((1));
> +
> +#if __GNUC_PREREQ (4,3) && defined __REDIRECT && defined __OPTIMIZE__
> +/* Issue a warning for use of gettimeofday with a non-null __tz argument.  */
> +__warndecl (__warn_gettimeofday_nonnull_timezone,
> +            "gettimeofday with non-null or non-constant timezone parameter;"
> +            " this is obsolete and inaccurate, use localtime instead");
> +
> +extern int __REDIRECT_NTH (__gettimeofday_alias,
> +                           (struct timeval *__restrict __tv,
> +                            void *__restrict __tz), gettimeofday)
> +  __nonnull ((1));
> +
> +/* The double cast below works around a limitation in __builtin_constant_p
> +   in all released versions of GCC (as of August 2019).
> +   See <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91554>.  */
> +__fortify_function int
> +__NTH (gettimeofday (struct timeval *__restrict __tv, void *__restrict __tz))
> +{
> +  if (! (__builtin_constant_p ((short) (__intptr_t) __tz) && __tz == 0))
> +    __warn_gettimeofday_nonnull_timezone ();
> +
> +  return __gettimeofday_alias (__tv, __tz);
> +}
> +#endif
>  
>  #ifdef __USE_MISC
>  /* Set the current time of day and timezone information.
> -   This call is restricted to the super-user.  */
> +   This call is restricted to the super-user.
> +   Setting the timezone in this way is obsolete, but we don't yet
> +   warn about it because it still has some uses for which there is
> +   no alternative.  */
>  extern int settimeofday (const struct timeval *__tv,
>  			 const struct timezone *__tz)
>       __THROW;
>