[LTP] arch_prctl: add ARCH_GET_FS, ARCH_GET_GS tests

[linuxtestproject.agent@gmail.com]

Hi Dmitry,

On Sat, 10 Oct 2026, Dmitry Chestnykh wrote:
> arch_prctl: add ARCH_GET_FS, ARCH_GET_GS tests

> Signed-off-by: Dmitry Chestnykh <dm.chestnykh@gmail.com>

The commit body lacks a rationale. Explain the coverage gap and why
instruction reads are used as the independent oracle.

> #ifdef HAVE_GETAUXVAL
> # include <sys/auxv.h>
> #endif
>
> static void setup(void)
> {
> 	unsigned long long v = 0;
>
> #ifdef AT_HWCAP2
> 	v = getauxval(AT_HWCAP2);
> #endif
> 	tag = v & HWCAP2_FSGSBASE ? true : false;
> }

Without getauxval support, v stays zero and run() incorrectly reports
that the CPU/kernel does not support the instructions. Guard the entire
test with HAVE_GETAUXVAL and use TST_TEST_TCONF() to report missing libc
support, or provide an auxiliary-vector fallback.

> #define READ_FS_BASE() ({                              \
> 	unsigned long long __res_fs;                       \

> #define READ_GS_BASE() ({                              \
> 	unsigned long long __res_gs;                       \

These double-underscore names are reserved to the implementation.
Rename them to fs_base and gs_base.

> static struct tcase {
> 	unsigned int idx;
> 	void (*test_fn)(void);
> } tcases[] = {
> 	{0, test_fs},
> 	{1, test_gs}
> };

The table dispatches to separate per-case test functions rather than
parameterizing a common comparison. Use one comparison handler with
entries selecting the ARCH_GET operation and base reader; keep
GS-specific preparation in a helper and remove the unused idx member.

Verdict - Needs revision

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