[PATCH] setpriv: remove dependency on libcap headers

[Ryan Harkin]

Hi Denys/Patrick,

On 7 July 2017 at 17:00, Denys Vlasenko <vda.linux at googlemail.com> wrote:
> On Fri, Jul 7, 2017 at 3:40 PM, Patrick Steinhardt <ps at pks.im> wrote:
>> The setpriv applet is including <sys/capability.h> for the capget(2) and
>> capset(2) functions. Unfortunately, this header is not provided by
>> linux-headers, but by libcap instead, which requires users to have them
>> installed. As these functions are not actually implemented by libcap
>> itself but instead provided by the libc, no additional linking is
>> required and as such the depenency went by unnoticed.
>>
>> To get rid of this requirement, we can instead use a direct syscall.
>> Taking a look at musl libc's implementation of these funcitions, they
>> are direct wrappers to `syscall(SYS_capset, ...)` and
>> `syscall(SYS_capget, ...)`. As such, it is trivial to reimplement them
>> ourselves via syscall(2).
>>
>> function                                             old     new   delta
>> getcaps                                              230     246     +16
>> setpriv_main                                        1260    1266      +6
>
> Evidently, this is not the same, since code size increased.
>
> I'm going with this instead:
>
> // #include <sys/capability.h>
> // This header is in libcap, but the functions are in libc.
> // Comment in the header says this above capset/capget:
> /* system calls - look to libc for function to system call mapping */
> extern int capset(cap_user_header_t header, cap_user_data_t data);
> extern int capget(cap_user_header_t header, const cap_user_data_t data);
> // so for bbox, let's just repeat the declarations.
> // This way, libcap needs not be installed in build environment.

I'd just finished testing Patrick's version when this came in, so I
retested. Both work fine for me, but I see it's already pushed
upstream, so no need for a Tested-By.

Thanks both for fixing it so quickly, I appreciate it.

Regards,
Ryan.