kernel: mips: fix init crash/bootloop on 64-bit systems

The malta subtargets for mips64 and mips64el fail to start the init process
at boot, resulting in a boot loop. The issue was raised and analyzed within
FS#3277. Investigation suggested code near the [vdso] memory area of the
process was long jumping into a region inaccessible to the process, e.g.

 init: - preinit -
 init: Launched preinit instance, pid=522
 do_page_fault(): sending SIGSEGV to init for invalid read access from 0000000000000360
 epc = 0000000000000360 in init[aaab42b000+4000]
 ra  = 000000fffee385e0 in
 Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
 Rebooting in 1 seconds..

Note the low-memory read access and epc are the same. Upstream kernel 5.6
included a relevant patch and discussion:

  * d3f703c4359f ("mips: vdso: fix 'jalr t9' crash in vdso code")

Disassembly of the failing kernel's vdso.so confirmed presence of the
telltale long jumps, e.g.:

00000000000007c0 <__vdso_clock_getres@@LINUX_2.6>:
 [...]
 7dc:   0320f809        jalr    t9
 [...]

Restore booting mips64/mips64el malta by backporting the above commit:
  * 310-v5.6-mips-vdso-fix-jalr-t9-crash-in-vdso-code.patch

Fixes: 54310a3 ("malta: add kernel 5.4 config")
Fixes: FS#3277
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=3277

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>

target/linux/generic/backport-5.4/310-v5.6-mips-vdso-fix-jalr-t9-crash-in-vdso-code.patch

@@ -0,0 +1,59 @@
+From d3f703c4359ff06619b2322b91f69710453e6b6d Mon Sep 17 00:00:00 2001
+From: Victor Kamensky <kamensky@cisco.com>
+Date: Tue, 11 Feb 2020 11:24:33 -0800
+Subject: [PATCH] mips: vdso: fix 'jalr t9' crash in vdso code
+
+Observed that when kernel is built with Yocto mips64-poky-linux-gcc,
+and mips64-poky-linux-gnun32-gcc toolchain, resulting vdso contains
+'jalr t9' instructions in its code and since in vdso case nobody
+sets GOT table code crashes when instruction reached. On other hand
+observed that when kernel is built mips-poky-linux-gcc toolchain, the
+same 'jalr t9' instruction are replaced with PC relative function
+calls using 'bal' instructions.
+
+The difference boils down to -mrelax-pic-calls and -mexplicit-relocs
+gcc options that gets different default values depending on gcc
+target triplets and corresponding binutils. -mrelax-pic-calls got
+enabled by default only in mips-poky-linux-gcc case. MIPS binutils
+ld relies on R_MIPS_JALR relocation to convert 'jalr t9' into 'bal'
+and such relocation is generated only if -mrelax-pic-calls option
+is on.
+
+Please note 'jalr t9' conversion to 'bal' can happen only to static
+functions. These static PIC calls use mips local GOT entries that
+are supposed to be filled with start of DSO value by run-time linker
+(missing in VDSO case) and they do not have dynamic relocations.
+Global mips GOT entries must have dynamic relocations and they should
+be prevented by cmd_vdso_check Makefile rule.
+
+Solution call out -mrelax-pic-calls and -mexplicit-relocs options
+explicitly while compiling MIPS vdso code. That would get correct
+and consistent between different toolchains behaviour.
+
+Reported-by: Bruce Ashfield <bruce.ashfield@gmail.com>
+Signed-off-by: Victor Kamensky <kamensky@cisco.com>
+Signed-off-by: Paul Burton <paulburton@kernel.org>
+Cc: linux-mips@vger.kernel.org
+Cc: Ralf Baechle <ralf@linux-mips.org>
+Cc: James Hogan <jhogan@kernel.org>
+Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
+Cc: richard.purdie@linuxfoundation.org
+---
+ arch/mips/vdso/Makefile | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/arch/mips/vdso/Makefile b/arch/mips/vdso/Makefile
+index aa89a41dc5dd..848baeaef1f8 100644
+--- a/arch/mips/vdso/Makefile
++++ b/arch/mips/vdso/Makefile
+@@ -33,6 +33,7 @@ endif
+ cflags-vdso := $(ccflags-vdso) \
+ 	$(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
+ 	-O3 -g -fPIC -fno-strict-aliasing -fno-common -fno-builtin -G 0 \
++	-mrelax-pic-calls -mexplicit-relocs \
+ 	-fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
+ 	$(call cc-option, -fno-asynchronous-unwind-tables) \
+ 	$(call cc-option, -fno-stack-protector)
+-- 
+2.17.1
+