arm64: alternative: patch alternatives in the vDSO

Make it possible to use alternatives in the vDSO, so that better
implementations can be used if possible.

Signed-off-by: Joey Gouly <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Vincenzo Frascino <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/include/asm/vdso.h

···
 	(void *)(vdso_offset_##name - VDSO_LBASE + (unsigned long)(base)); \
 })
 
+extern char vdso_start[], vdso_end[];
+extern char vdso32_start[], vdso32_end[];
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* __ASM_VDSO_H */

arch/arm64/kernel/alternative.c

···
 
 #include <linux/init.h>
 #include <linux/cpu.h>
+#include <linux/elf.h>
 #include <asm/cacheflush.h>
 #include <asm/alternative.h>
 #include <asm/cpufeature.h>
 #include <asm/insn.h>
+#include <asm/module.h>
 #include <asm/sections.h>
+#include <asm/vdso.h>
 #include <linux/stop_machine.h>
 
 #define __ALT_PTR(a, f)		((void *)&(a)->f + (a)->f)
···
 	}
 }
 
+void apply_alternatives_vdso(void)
+{
+	struct alt_region region;
+	const struct elf64_hdr *hdr;
+	const struct elf64_shdr *shdr;
+	const struct elf64_shdr *alt;
+	DECLARE_BITMAP(all_capabilities, ARM64_NPATCHABLE);
+
+	bitmap_fill(all_capabilities, ARM64_NPATCHABLE);
+
+	hdr = (struct elf64_hdr *)vdso_start;
+	shdr = (void *)hdr + hdr->e_shoff;
+	alt = find_section(hdr, shdr, ".altinstructions");
+	if (!alt)
+		return;
+
+	region = (struct alt_region){
+		.begin	= (void *)hdr + alt->sh_offset,
+		.end	= (void *)hdr + alt->sh_offset + alt->sh_size,
+	};
+
+	__apply_alternatives(&region, false, &all_capabilities[0]);
+}
+
 /*
  * We might be patching the stop_machine state machine, so implement a
  * really simple polling protocol here.
···
 
 void __init apply_alternatives_all(void)
 {
+	apply_alternatives_vdso();
 	/* better not try code patching on a live SMP system */
 	stop_machine(__apply_alternatives_multi_stop, NULL, cpu_online_mask);
 }

arch/arm64/kernel/vdso.c

···
 #include <asm/signal32.h>
 #include <asm/vdso.h>
 
-extern char vdso_start[], vdso_end[];
-extern char vdso32_start[], vdso32_end[];
-
 enum vdso_abi {
 	VDSO_ABI_AA64,
 	VDSO_ABI_AA32,

arch/arm64/kernel/vdso/vdso.lds.S

···
 	PROVIDE (_etext = .);
 	PROVIDE (etext = .);
 
+	. = ALIGN(4);
+	.altinstructions : {
+		__alt_instructions = .;
+		*(.altinstructions)
+		__alt_instructions_end = .;
+	}
+
 	.dynamic	: { *(.dynamic) }		:text	:dynamic
 
 	.rela.dyn	: ALIGN(8) { *(.rela .rela*) }