Rationale for setting COMMLEN to 19 in struct kinfo_proc

[Brooks Davis]

On Tue, May 29, 2018 at 11:42:47PM +0300, Gleb Popov wrote:
> Hello.
> 
> I've been debugging a failing Qt (KDE, to be precise) test and found that
> process name returned in kinfo_proc structure gets truncated to 19 symbols.
> This causes other errors in the application. I'm wondering why is this
> field so small and is there possibility to increase it? I think, having
> applications with names longer than 19 characters is not that rare.
> 
> Relevant header: /usr/include/sys/user.h
> 
> Simple testcase to feature the problem:
> 
> # cat k.cpp
> #include <sys/types.h>
> #include <signal.h>
> #include <unistd.h>
> #include <stdio.h>
> 
> #include <sys/cdefs.h>
> #include <sys/param.h>
> #include <sys/sysctl.h>
> #include <sys/user.h>
> 
> int main()
> {
>     auto pid = getpid();
>     struct kinfo_proc kp;
>     int mib[4] = { CTL_KERN, KERN_PROC, KERN_PROC_PID, (int)pid };
> 
>     size_t len = sizeof(kp);
>     u_int mib_len = sizeof(mib)/sizeof(u_int);
> 
>     if (sysctl(mib, mib_len, &kp, &len, NULL, 0) < 0)
>         return -1;
> 
>     puts(kp.ki_tdname);
>     puts(kp.ki_comm);
> 
>     return 0;
> }
> # c++ -o abcdefghijklmnopqrstuvwxyz
> # ./abcdefghijklmnopqrstuvwxyz
> abcdefghijklmnop
> abcdefghijklmnopqrs

Changing the size of fields in kinfo_proc would be enormously
disruptive.  Take a look at all the source that uses KERN_PROC_PID for a
subset of things that would break.

Sometimes we can break these interfaces, but I think this one would
require new sysctl mibs.  If we did that we should come up with an
interface that is identical between 32-bit and 32-bit systems and
doesn't export pointers.

-- Brooks
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