LoongArch: Fix and simplify fcsr initialization on execve()

There has been a lingering bug in LoongArch Linux systems causing some
GCC tests to intermittently fail (see Closes link). I've made a minimal
reproducer:

zsh% cat measure.s
.align 4
.globl _start
_start:
movfcsr2gr $a0, $fcsr0
bstrpick.w $a0, $a0, 16, 16
beqz $a0, .ok
break 0
.ok:
li.w $a7, 93
syscall 0
zsh% cc mesaure.s -o measure -nostdlib
zsh% echo $((1.0/3))
0.33333333333333331
zsh% while ./measure; do ; done

This while loop should not stop as POSIX is clear that execve must set
fenv to the default, where FCSR should be zero. But in fact it will
just stop after running for a while (normally less than 30 seconds).
Note that "$((1.0/3))" is needed to reproduce this issue because it
raises FE_INVALID and makes fcsr0 non-zero.

The problem is we are currently relying on SET_PERSONALITY2() to reset
current->thread.fpu.fcsr. But SET_PERSONALITY2() is executed before
start_thread which calls lose_fpu(0). We can see if kernel preempt is
enabled, we may switch to another thread after SET_PERSONALITY2() but
before lose_fpu(0). Then bad thing happens: during the thread switch
the value of the fcsr0 register is stored into current->thread.fpu.fcsr,
making it dirty again.

The issue can be fixed by setting current->thread.fpu.fcsr after
lose_fpu(0) because lose_fpu() clears TIF_USEDFPU, then the thread
switch won't touch current->thread.fpu.fcsr.

The only other architecture setting FCSR in SET_PERSONALITY2() is MIPS.
I've ran a similar test on MIPS with mainline kernel and it turns out
MIPS is buggy, too. Anyway MIPS do this for supporting different FP
flavors (NaN encodings, etc.) which do not exist on LoongArch. So for
LoongArch, we can simply remove the current->thread.fpu.fcsr setting
from SET_PERSONALITY2() and do it in start_thread(), after lose_fpu(0).

The while loop failing with the mainline kernel has survived one hour
after this change on LoongArch.

Fixes: 803b0fc5c3f2baa ("LoongArch: Add process management")
Closes: https://github.com/loongson-community/discussions/issues/7
Link: https://lore.kernel.org/linux-mips/7a6aa1bbdbbe2e63ae96ff163fab0349f58f1b9e.camel@xry111.site/
Cc: stable@vger.kernel.org
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Preserve syscall nr across execve()

Currently, we store syscall nr in pt_regs::regs[11] and syscall execve()
accidentally overrides it during its execution:

sys_execve()
-> do_execve()
-> do_execveat_common()
-> bprm_execve()
-> exec_binprm()
-> search_binary_handler()
-> load_elf_binary()
-> ELF_PLAT_INIT()

ELF_PLAT_INIT() reset regs[11] to 0, so in syscall_exit_to_user_mode()
we later get a wrong syscall nr. This breaks tools like execsnoop since
it relies on execve() tracepoints.

Skip pt_regs::regs[11] reset in ELF_PLAT_INIT() to fix the issue.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Define relocation types for ABI v2.10

The relocation types from 101 to 109 are used by GNU binutils >= 2.41,
add their definitions to use them in later patches.

Link: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=include/elf/loongarch.h#l230
Cc: <stable@vger.kernel.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Define ELF relocation types added in ABIv2.0

These relocation types are used by GNU binutils >= 2.40 and GCC >= 13.
Add their definitions so we will be able to use them in later patches.

Link: https://github.com/loongson/LoongArch-Documentation/pull/57
Tested-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Disable executable stack by default

Disable executable stack for LoongArch by default, as all modern
architectures do.

Reported-by: Andreas Schwab <schwab@suse.de>
Suggested-by: WANG Xuerui <git@xen0n.name>
Link: https://sourceware.org/pipermail/binutils/2022-July/121992.html
Tested-by: WANG Xuerui <git@xen0n.name>
Tested-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Add ELF and module support

Add ELF-related definition and module relocation code for basic
LoongArch support.

Cc: Jessica Yu <jeyu@kernel.org>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>