Libguestfs - Feb 2023 - [libnbd PATCH v3 07/29] lib/utils: add async-signal-safe assert()

On Wed, Feb 15, 2023 at 03:11:36PM +0100, Laszlo Ersek
wrote:
> Add an assert() variant that we may call between fork() and exec*().
> 
> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
> ---
> 
> +++ b/lib/internal.h
> +
> +#ifdef NDEBUG
> +#define NBD_INTERNAL_FORK_SAFE_ASSERT(expression) ((void)0)
May be affected by outcome of side discussion on 01/29 about whether
we want space after cast.
> +#else
> +#define NBD_INTERNAL_FORK_SAFE_ASSERT(expression)                        \
> +  (nbd_internal_fork_safe_assert ((expression) != 0, __FILE__, __LINE__, \
> +                                  __func__, #expression))
> +#endif
> +
>  #endif /* LIBNBD_INTERNAL_H */
> +++ b/lib/utils.c
> @@ -431,13 +431,35 @@ nbd_internal_socketpair (int domain, int type, int
protocol, int *fds)
>    if (ret == 0) {
>      for (i = 0; i < 2; i++) {
>        if (fcntl (fds[i], F_SETFD, FD_CLOEXEC) == -1) {
>          close (fds[0]);
>          close (fds[1]);
>          return -1;
>        }
>      }
>    }
>  #endif
>  
>    return ret;
>  }
> +
> +void
> +nbd_internal_fork_safe_assert (int result, const char *file, long line,
> +                               const char *func, const char *assertion)
> +{
> +  const char *line_out;
> +  char line_buf[32];
> +
> +  if (result)
> +    return;
Since no code should ever directly call
nbd_internal_fork_safe_assert(0,...), but instead go through our macro
that has already filtered on expression's value, this conditional is
technically dead code; but I'm okay whether it stays in or gets
removed.
> +
> +  xwrite (STDERR_FILENO, file, strlen (file));
> +  xwrite (STDERR_FILENO, ":", 1);
Presumably, if our first best-effort xwrite() fails to produce full
output, all later xwrite() will hopefully hit the same error condition
so that we aren't producing even more-mangled output where later
syscalls succeed despite missing context earlier in the overall
output.  If it were something we truly wanted to worry about, the
solution would be pre-loading the entire message into a single buffer,
then calling xwrite() just once - but that's far more effort for
something we don't anticipate hitting in normal usage anyways.  I'm
happy if you ignore this whole paragraph of mine.
> +  line_out = nbd_internal_fork_safe_itoa (line, line_buf, sizeof
line_buf);
> +  xwrite (STDERR_FILENO, line_out, strlen (line_out));
> +  xwrite (STDERR_FILENO, ": ", 2);
> +  xwrite (STDERR_FILENO, func, strlen (func));
> +  xwrite (STDERR_FILENO, ": Assertion `", 13);
> +  xwrite (STDERR_FILENO, assertion, strlen (assertion));
> +  xwrite (STDERR_FILENO, "' failed.\n", 10);
These days, the quoting style `' seems to be disappearing in favor of
''.  But a quick test showed me that at least glibc 2.36 is still
producing the message with the quotes as you have spelled it here.
> +  abort ();
Huh. I checked the source to glibc's assert.c, where it includes a
comment about having to work cleanly even if a second assertion ends
up being raised from within a SIGABRT handler active during the first
attempt to abort().  But POSIX says that abort() shall attempt to
override any SIGABRT handler in place, so I wonder if glibc's
implementation is a bit too defensive.  At any rate, your
implementation looks reasonable, and more to the point, satisfies your
desire of being async-signal-safe and thus appropriate between fork()
and exec().

Reviewed-by: Eric Blake <eblake at redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

On 2/15/23 21:57, Eric Blake wrote:
> On Wed, Feb 15, 2023 at 03:11:36PM +0100, Laszlo Ersek wrote:
>> Add an assert() variant that we may call between fork() and exec*().
>>
>> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
>> ---
>>
> 
>> +++ b/lib/internal.h
> 
>> +
>> +#ifdef NDEBUG
>> +#define NBD_INTERNAL_FORK_SAFE_ASSERT(expression) ((void)0)
> 
> May be affected by outcome of side discussion on 01/29 about whether
> we want space after cast.
> 
>> +#else
>> +#define NBD_INTERNAL_FORK_SAFE_ASSERT(expression)                     
\
>> +  (nbd_internal_fork_safe_assert ((expression) != 0, __FILE__,
__LINE__, \
>> +                                  __func__, #expression))
>> +#endif
>> +
>>  #endif /* LIBNBD_INTERNAL_H */
> 
>> +++ b/lib/utils.c
>> @@ -431,13 +431,35 @@ nbd_internal_socketpair (int domain, int type,
int protocol, int *fds)
>>    if (ret == 0) {
>>      for (i = 0; i < 2; i++) {
>>        if (fcntl (fds[i], F_SETFD, FD_CLOEXEC) == -1) {
>>          close (fds[0]);
>>          close (fds[1]);
>>          return -1;
>>        }
>>      }
>>    }
>>  #endif
>>  
>>    return ret;
>>  }
>> +
>> +void
>> +nbd_internal_fork_safe_assert (int result, const char *file, long
line,
>> +                               const char *func, const char
*assertion)
>> +{
>> +  const char *line_out;
>> +  char line_buf[32];
>> +
>> +  if (result)
>> +    return;
> 
> Since no code should ever directly call
> nbd_internal_fork_safe_assert(0,...), but instead go through our macro
> that has already filtered on expression's value,
I either don't understand, or I disagree. With NDEBUG not defined, the
NBD_INTERNAL_FORK_SAFE_ASSERT macro is always expanded to a call to
nbd_internal_fork_safe_assert(). And the expression to check is
evaluated in the first argument of that function call:

nbd_internal_fork_safe_assert ((expression) != 0, ...

So I think the early return is definitely necessary here.
> this conditional is
> technically dead code; but I'm okay whether it stays in or gets
> removed.
> 
>> +
>> +  xwrite (STDERR_FILENO, file, strlen (file));
>> +  xwrite (STDERR_FILENO, ":", 1);
> 
> Presumably, if our first best-effort xwrite() fails to produce full
> output, all later xwrite() will hopefully hit the same error condition
> so that we aren't producing even more-mangled output where later
> syscalls succeed despite missing context earlier in the overall
> output.  If it were something we truly wanted to worry about, the
> solution would be pre-loading the entire message into a single buffer,
> then calling xwrite() just once - but that's far more effort for
> something we don't anticipate hitting in normal usage anyways.  I'm
> happy if you ignore this whole paragraph of mine.
Any single buffer presents the problem of sizing the buffer
appropriately, which we can't do in this context :)
> 
>> +  line_out = nbd_internal_fork_safe_itoa (line, line_buf, sizeof
line_buf);
>> +  xwrite (STDERR_FILENO, line_out, strlen (line_out));
>> +  xwrite (STDERR_FILENO, ": ", 2);
>> +  xwrite (STDERR_FILENO, func, strlen (func));
>> +  xwrite (STDERR_FILENO, ": Assertion `", 13);
>> +  xwrite (STDERR_FILENO, assertion, strlen (assertion));
>> +  xwrite (STDERR_FILENO, "' failed.\n", 10);
> 
> These days, the quoting style `' seems to be disappearing in favor of
> ''.  But a quick test showed me that at least glibc 2.36 is still
> producing the message with the quotes as you have spelled it here.
Personally I dislike `', and like '' (and ""), but I
specifically
modeled the message on the standard assert() failure message on RHEL9.
> 
>> +  abort ();
> 
> Huh. I checked the source to glibc's assert.c, where it includes a
> comment about having to work cleanly even if a second assertion ends
> up being raised from within a SIGABRT handler active during the first
> attempt to abort().  But POSIX says that abort() shall attempt to
> override any SIGABRT handler in place, so I wonder if glibc's
> implementation is a bit too defensive.
What I did was, I checked POSIX on assert(), and the spec there simply
says that assert() calls abort():

"When it is executed, if expression (which shall have a scalar type) is
false (that is, compares equal to 0), assert() shall write information
about the particular call that failed on stderr and shall call abort()."
> At any rate, your
> implementation looks reasonable, and more to the point, satisfies your
> desire of being async-signal-safe and thus appropriate between fork()
> and exec().
> 
> Reviewed-by: Eric Blake <eblake at redhat.com>
> 
Thanks!
Laszlo