[LTP] arch_prctl: add ARCH_GET_FS, ARCH_GET_GS tests

[linuxtestproject.agent@gmail.com]

Hi Dmitry,

On Fri Oct 9 18:52:24 2026 +0300, Dmitry Chestnykh wrote:
> arch_prctl: add ARCH_GET_FS, ARCH_GET_GS tests

> +++ b/testcases/kernel/syscalls/arch_prctl/arch_prctl02.c
> +	unsigned long gs = READ_GS_BASE();
> +		TST_EXP_EQ_LU(gs_prctl, gs);

The default GS base is zero in ordinary runs, so an always-zero getter
passes; replacing ARCH_GET_GS with a successful zero write reproduced this.
Also exercise a known nonzero 64-bit GS base and restore the original
afterward, without changing the libc-owned FS base.

> +	TST_EXP_PASS_SILENT(arch_prctl_get(ARCH_GET_FS, &fs_prctl));
> +	if (TST_PASS)
> +		TST_EXP_EQ_LU(fs_prctl, fs);
> +	else
> +		tst_res(TFAIL, "got wrong fs register value");

TST_EXP_PASS_SILENT() already reports syscall failures; injecting EINVAL
for both getters produces four failures for two calls. Remove both else
branches: they duplicate the failures and claim value mismatches without
comparing either value.

> +static void run(unsigned int index)
> +{
> +	(void)index;
> +	.test = run,
> +	.tcnt = 1,

Represent FS and GS as two tcases entries and use
.tcnt = ARRAY_SIZE(tcases), with the index selecting the opcode and
corresponding register read instead of duplicating both cases in run(0).

> +    unsigned long v = 0;
> +	unsigned long long __res;                          \

Use a tab for v's indentation; checkpatch reports a warning here.
Rename __res to a non-reserved identifier in both register-reading macros.

> +/*\
> + *  Simple test on arch_prctl to get FS_BASE and GS_BASE
> + */

Use the :manpage:`arch_prctl(2)` role and describe the actual assertion:
``ARCH_GET_FS`` and ``ARCH_GET_GS`` return the bases read independently
by RDFSBASE and RDGSBASE.

Verdict - Needs revision

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