riscv: 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: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230128172856.3814-11-jszhang@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/vdso.h

@@ -28,8 +28,12 @@
 #define COMPAT_VDSO_SYMBOL(base, name)						\
 	(void __user *)((unsigned long)(base) + compat__vdso_##name##_offset)
 
+extern char compat_vdso_start[], compat_vdso_end[];
+
 #endif /* CONFIG_COMPAT */
 
+extern char vdso_start[], vdso_end[];
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* CONFIG_MMU */

arch/riscv/kernel/alternative.c

@@ -11,7 +11,9 @@
 #include <linux/cpu.h>
 #include <linux/uaccess.h>
 #include <asm/alternative.h>
+#include <asm/module.h>
 #include <asm/sections.h>
+#include <asm/vdso.h>
 #include <asm/vendorid_list.h>
 #include <asm/sbi.h>
 #include <asm/csr.h>
@@ -160,6 +162,31 @@ static void __init_or_module _apply_alternatives(struct alt_entry *begin,
 				stage);
 }
 
+#ifdef CONFIG_MMU
+static void __init apply_vdso_alternatives(void)
+{
+	const Elf_Ehdr *hdr;
+	const Elf_Shdr *shdr;
+	const Elf_Shdr *alt;
+	struct alt_entry *begin, *end;
+
+	hdr = (Elf_Ehdr *)vdso_start;
+	shdr = (void *)hdr + hdr->e_shoff;
+	alt = find_section(hdr, shdr, ".alternative");
+	if (!alt)
+		return;
+
+	begin = (void *)hdr + alt->sh_offset,
+	end = (void *)hdr + alt->sh_offset + alt->sh_size,
+
+	_apply_alternatives((struct alt_entry *)begin,
+			    (struct alt_entry *)end,
+			    RISCV_ALTERNATIVES_BOOT);
+}
+#else
+static void __init apply_vdso_alternatives(void) { }
+#endif
+
 void __init apply_boot_alternatives(void)
 {
 	/* If called on non-boot cpu things could go wrong */
@@ -168,6 +195,8 @@ void __init apply_boot_alternatives(void)
 	_apply_alternatives((struct alt_entry *)__alt_start,
 			    (struct alt_entry *)__alt_end,
 			    RISCV_ALTERNATIVES_BOOT);
+
+	apply_vdso_alternatives();
 }
 
 /*

arch/riscv/kernel/vdso.c

@@ -22,11 +22,6 @@ struct vdso_data {
 };
 #endif
 
-extern char vdso_start[], vdso_end[];
-#ifdef CONFIG_COMPAT
-extern char compat_vdso_start[], compat_vdso_end[];
-#endif
-
 enum vvar_pages {
 	VVAR_DATA_PAGE_OFFSET,
 	VVAR_TIMENS_PAGE_OFFSET,

arch/riscv/kernel/vdso/vdso.lds.S

@@ -40,6 +40,13 @@ SECTIONS
 	. = 0x800;
 	.text		: { *(.text .text.*) }		:text
 
+	. = ALIGN(4);
+	.alternative : {
+		__alt_start = .;
+		*(.alternative)
+		__alt_end = .;
+	}
+
 	.data		: {
 		*(.got.plt) *(.got)
 		*(.data .data.* .gnu.linkonce.d.*)