seccomp.2: Remove unneeded cast

From the email discussion:

> Hi Alex,
>
> On 9/25/20 9:31 AM, Alejandro Colomar wrote:
>> Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
>> ---
>>   man2/seccomp.2 | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/man2/seccomp.2 b/man2/seccomp.2
>> index 58033da1c..d6b856c32 100644
>> --- a/man2/seccomp.2
>> +++ b/man2/seccomp.2
>> @@ -1101,7 +1101,7 @@ install_filter(int syscall_nr, int t_arch, int f_errno)
>>       };
>>
>>       struct sock_fprog prog = {
>> -        .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
>> +        .len = sizeof(filter) / sizeof(filter[0]),
>>           .filter = filter,
>>       };
>
> I have a small doubt about this change. With the change,
> there are no compilation warnings.
>
> But, if we change the code to something slightly different:
>
> [[
>      size_t x = (sizeof(filter) / sizeof(filter[0]));
>      struct sock_fprog prog = {
>          .len = x,
>          .filter = filter,
>      };
> ]]
>
> The "cc -Wconversion" gives us the following warning:
>
>      warning: conversion from ‘size_t’ {aka ‘long unsigned int’}
>      to ‘short unsigned int’ may change value
>
> Presumably we don't get a warning for an assignment of the form
>
>      .len = (sizeof(filter) / sizeof(filter[0]))
>
> because the compiler is smart enough to work out that the
> value of the constant expression is within the range of
> "unsigned short".
>
> Your thoughts?

Hi Michael,

I'd say that the cast doesn't fix any problems at all.  It silences a
valid warning, and I'd use a pragma for that (to be more explicit about
the intention of silencing a warning) if I do want -Wconversion enabled
(which usually I don't want, because it's too noisy) and I'm sure that
this won't overflow.  I'd limit the use casts to only when I *really*
need to.

I guess that if you enable -O3, the warning will vanish again because
the compiler will optimize away 'x' (but I didn't test).

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man2/seccomp.2 b/man2/seccomp.2
index 58033da1c45d5269be080c2b755d323a97867753..d6b856c326dedd4888c78a84ab2125cc9b660c11 100644 (file)
--- a/man2/seccomp.2
+++ b/man2/seccomp.2
@@ -1101,7 +1101,7 @@ install_filter(int syscall_nr, int t_arch, int f_errno)
     };
 
     struct sock_fprog prog = {
-        .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
+        .len = sizeof(filter) / sizeof(filter[0]),
         .filter = filter,
     };