[LTP] arch_prctl: add ARCH_GET_FS, ARCH_GET_GS tests

[linuxtestproject.agent@gmail.com]

Hi Dmitry,

On Thu, 8 Oct 2026, Dmitry Chestnykh wrote:
> arch_prctl: add ARCH_GET_FS, ARCH_GET_GS tests

> +CFLAGS += -mfsgsbase

This directory-wide x86 option breaks supported non-x86 builds, including
the existing arch_prctl01 target; AArch64 clang rejects it. Remove the
option: the new test's inline assembly compiles without it on x86_64.

> +	asm volatile("rdfsbase %0" : "=r"(__res));         \
> +	asm volatile("rdgsbase %0" : "=r"(__res));         \
> +	.supported_archs = (const char *const []){"x86_64", NULL}

Removing the compiler option alone is insufficient: the unconditional
assembly still fails on AArch64, and lapi/arch_prctl.h supplies no
ARCH_GET_FS/ARCH_GET_GS definitions on non-x86 targets. The supported_archs
check runs only after compilation; provide a buildable unsupported-feature
path or filter the target appropriately in the Makefile.

> +#if defined(__has_include) && __has_include(<asm/hwcap2.h>)

GCC 4.8.5, a supported compiler, cannot parse this expression: the left
operand does not protect an undefined function-like preprocessing operator.
The repository's sparse check also rejects this line. Use separately nested
checks or configured optional-header inclusion.

> +#include <asm/hwcap2.h>
> +#else
> +#warning "Cannot obtain HWCAP2_FSGSBASE from system headers"
> +#define HWCAP2_FSGSBASE (1UL << 1)

An existing older header bypasses the fallback even when it lacks this bit:
compilation against Linux 5.4's hwcap2.h fails with HWCAP2_FSGSBASE
undeclared.
Apply the fallback after optional header inclusion, independently of header
availability, and put the compatibility definition in include/lapi.

> +/arch_prctl02

The new binary has no entry in any runtest file, so normal syscall-suite
runs never execute it. Add "arch_prctl02 arch_prctl02" to runtest/syscalls.

> +	TEST(arch_prctl_get(ARCH_GET_FS, &fs_prctl));
> +	if (TST_RET == 0 && fs_prctl == fs)
> +		tst_res(TPASS, "get fs succeed");
> +	else
> +		tst_res(TFAIL, "got wrong fs register value");

These messages discard errno and the expected/actual bases, and report a
syscall failure as a register mismatch. Use TST_EXP_PASS_SILENT(), then
TST_EXP_EQ_LU() only when TST_PASS is true; apply the same change to GS.

> + * Author: Dmitry Chestnykh <dm.chestnykh@gmail.com>

Add the required copyright line with year and author, for example
"Copyright (c) 2026 Dmitry Chestnykh <dm.chestnykh@gmail.com>".

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

Use the required manpage role and describe the comparison, for example:
"Check that :manpage:`arch_prctl(2)` with ``ARCH_GET_FS`` and ``ARCH_GET_GS``
returns bases matching ``rdfsbase`` and ``rdgsbase``."

Verdict - Needs revision

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