x/sys/unix: Utsname.Version is too small?

[seeplusplus]

Go version

go1.20.12 freebsd/386

Output of go env in your module/workspace:

Not relevant.

What did you do?

1. Built the following minimal reproducible Go program targeting FreeBSD, shown here:
   FreeBSD: couldn't get uname: cannot allocate memory prometheus/node_exporter#2809 (comment)
2. Copied the file to a running instances of pfSense
3. Ran it
4. Got an "Cannot allocate memory" error
5. Cloned sys/unix locally and played around with the syscall_freebsd.go file, in particular I commented out sections of the Uname method until I found the section that caused the error. That was this section:

 mib = []_C_int{CTL_KERN, KERN_VERSION}
          n = unsafe.Sizeof(uname.Version)
          if err := sysctl(mib, &uname.Version[0], &n, nil, 0); err != nil {
                 return err
}   

Upon further digging I found that uname.Version is set to 256 bytes here:

https://cs.opensource.google/go/x/sys/+/master:unix/ztypes_freebsd_amd64.go;l=639-645;bpv=0?q=Utsname&ss=go%2Fx%2Fsys

so then I,
6. Modified the above locally to a larger value, 1024.
7. Rebuilt the test binary with all code from step 5 uncommented.
8. Reran the binary on the pfSense OS,
9. No error.

What did you see happen?

Got "cannot allocate memory" until I made the Utsname.Version field larger.

What did you expect to see?

The Uname to be returned.

Apologies if this isn't the correct venue for this report. I'm not a Go developer, nor have I ever worked on FreeBSD or pfSense. I am willing to do what's needed to fix this, but I'm not sure exactly what's "wrong" here. I know the types_freebsd.go file is auto-generated, but I'm not familiar with the domain to know if:

1. pfSense is errantly reporting a KERN_VERSION larger than 256 bytes, or
2. the types_freebsd.go file needs to be regenerated/modified in some way.

Edit: Fwiw I was doing some reading and it looks like maybe this isn't the first time someone has encountered this issue in FreeBSD?

https://github.com/freebsd/freebsd-src/blob/82bebc7936581e9c4ff3604d4cb998f8cc017f50/sys/kern/kern_xxx.c#L351-L371

[bcmills]

(attn @golang/freebsd)

[paulzhol]

Both FreeBSD 13 and 14 libc use 256 bytes for utsname struct fields (Or 32 bytes depending if the __KERNEL__ macro value is defined).
The underlying implementation via __xuname(int namesize, void *namebuf) however is variable size:

https://cgit.freebsd.org/src/tree/sys/sys/utsname.h?h=stable/13
https://cgit.freebsd.org/src/tree/lib/libc/gen/uname.c?h=stable/13
https://cgit.freebsd.org/src/tree/lib/libc/gen/__xuname.c?h=stable/13

https://cgit.freebsd.org/src/tree/sys/sys/utsname.h?h=stable/14
https://cgit.freebsd.org/src/tree/lib/libc/gen/uname.c?h=stable/14
https://cgit.freebsd.org/src/tree/lib/libc/gen/__xuname.c?h=stable/14

This is also true for pFSense AFIK: https://github.com/pfsense/FreeBSD-src/blob/RELENG_2_7_2/sys/sys/utsname.h

[paulzhol]

The following C reproducer returns ENOMEM on pfSense CE 2.7.2, while successful on FreeBSD 14.0-RELEASE:

#include <sys/utsname.h>
#include <sys/cdefs.h>
#include <sys/param.h>
#include <sys/sysctl.h>
#include <errno.h>
#include <stdlib.h>

int main(int argc, char **argv) {
        struct utsname u = {0};
        int mib[2], rval;
        size_t len;
        mib[0] = CTL_KERN;
        mib[1] = KERN_VERSION;
        len = sizeof u.version;
        char* q = (char *)u.version;
        rval = sysctl(mib, 2, q, &len, NULL, 0);
        printf("rval: %d, errno: %d\n", rval, errno);
        printf("u.version: %s\n", &u.version[0]);
        return 0;
}

rval: -1, errno: 12
u.version: FreeBSD 14.0-CURRENT amd64 1400094 #1 RELENG_2_7_2-n255948-8d2b56da39c: Wed Dec  6 20:45:47 UTC 2023
    root@freebsd:/var/jenkins/workspace/pfSense-CE-snapshots-2_7_2-main/obj/amd64/StdASW5b/var/jenkins/workspace/pfSense-CE-snapshots-2_7_2-main/sources/Fr

rval: 0, errno: 0
u.version: FreeBSD 14.0-RELEASE-p3 #0: Mon Dec 11 04:56:01 UTC 2023
    root@amd64-builder.daemonology.net:/usr/obj/usr/src/amd64.amd64/sys/GENERIC

Looks like we should suppress ENOMEM and truncate to match the __xuname behavior in eg:
https://github.com/golang/sys/blob/2f2cc5dae4e240ccd9b4142205a71a311328419f/unix/syscall_freebsd.go#L190-L192

[gopherbot]

Change https://go.dev/cl/562617 mentions this issue: unix: supress ENOMEM errors from sysctl's implementing Uname(uname *Utsname) due to truncated fields.

[seeplusplus]

Change https://go.dev/cl/562617 mentions this issue: unix: supress ENOMEM errors from sysctl's implementing Uname(uname *Utsname) due to truncated fields.

@paulzhol I have tested the above patch on my pfSense machine, this works, thank you!