proposal: syscall: export padding field names on linux/s390x consistent with other arches

[bcmills]

While investigating build reproducibility for the Go toolchain, we found that at least one distribution is patching syscall to change a padding field named _ to the X__… name used on other platforms. The patch was originally written for #18628 as https://go.dev/cl/35262.

#18632 was filed for the equivalent fields in x/sys/unix, and it was decided to unexport them on other platforms instead. However, syscall remains inconsistent, and the patch was dropped from the Go repo as unnecessary.

The existence of the downstream patch suggests to me that some Go user found it necessary after all.

To improve consistency and reduce the need for downstream patching, I propose that we export the syscall.IfInfomsg.X__ifi_pad field on linux/s390x, along with any other syscall struct fields that are similarly already exported on the other linux architectures, if any exist. (I will do a quick analysis to check.)

[rsc]

Making these uniform makes sense even though syscall is frozen, and it removes an upstream patch.

[bcmills]

IfInfomsg.X__ifi_pad may be the only field needed for consistency, although we may also consider Sysinfo_t.X_f.

---

I checked the other exported X__ fields, and found that of the 13 GOARCH values supported by syscall:

• Fsid.X__val is already exported on all 13.
• IfInfomsg.X__ifi_pad is exported on 12 of the 13 (all but s390x).
• Sysinfo_t.X_f is exported on all but s390x and riscv64.

The remaining fields are inconsistent across architectures:

Field	GOARCH
X_BASIC_STATS	loong64
X_f	386, amd64, arm64, arm, loong64, mips64, mips64le, mips, mipsle, ppc64, ppc64le
X__glibc_reserved	arm64, loong64, ppc64le, riscv64
X__glibc_reserved4	ppc64le
X__glibc_reserved5	ppc64le
X__glibc_reserved6	ppc64le
X__ifi_pad	386, amd64, arm64, arm, loong64, mips64, mips64le, mips, mipsle, ppc64, ppc64le, riscv64
X__pad0	amd64
X__pad1	386, arm64, arm, loong64, riscv64
X__pad2	386, arm64, arm, loong64, ppc64, ppc64le, riscv64
X_padFd	loong64, ppc64, ppc64le
X__reserved	loong64
X__spare0	loong64
X__spare2	loong64
X__spare3	loong64
X__st_ino	386, arm
X__unused	amd64, ppc64
X__unused4	ppc64
X__unused5	ppc64
X__unused6	ppc64
X__val	386, amd64, arm64, arm, loong64, mips64, mips64le, mips, mipsle, ppc64, ppc64le, riscv64, s390x

[bcmills]

(CC @golang/s390x @golang/riscv64)

[rsc]

Accepted as trivial.

[gopherbot]

Change https://go.dev/cl/457557 mentions this issue: syscall: export X__ifi_pad and X_f on s390x and riscv64