Expand the elf brandelf infrastructure to give access to the whole ELF

header (Elf_Ehdr) to determine if a particular interpretor wants to
accept it or not. Use this mechanism to filter EABI arm on OABI arm
kernels, and vice versa. This method could also be used to implement
OABI on EABI arm kernels, if desired, or to allow a single mips kernel
to run o32, n32 and n64 binaries.

Differential Revision: https://reviews.freebsd.org/D609

sys/arm/arm/elf_machdep.c

@@ -46,6 +46,8 @@ __FBSDID("$FreeBSD$");
 #include <machine/elf.h>
 #include <machine/md_var.h>
 
+static boolean_t elf32_arm_abi_supported(struct image_params *);
+
 struct sysentvec elf32_freebsd_sysvec = {
 	.sv_size	= SYS_MAXSYSCALL,
 	.sv_table	= sysent,
@@ -90,7 +92,8 @@ static Elf32_Brandinfo freebsd_brand_info = {
 	.sysvec		= &elf32_freebsd_sysvec,
 	.interp_newpath	= NULL,
 	.brand_note	= &elf32_freebsd_brandnote,
-	.flags		= BI_CAN_EXEC_DYN | BI_BRAND_NOTE
+	.flags		= BI_CAN_EXEC_DYN | BI_BRAND_NOTE,
+	.header_supported= elf32_arm_abi_supported,
 };
 
 SYSINIT(elf32, SI_SUB_EXEC, SI_ORDER_FIRST,
@@ -106,13 +109,42 @@ static Elf32_Brandinfo freebsd_brand_oinfo = {
 	.sysvec		= &elf32_freebsd_sysvec,
 	.interp_newpath	= NULL,
 	.brand_note	= &elf32_freebsd_brandnote,
-	.flags		= BI_CAN_EXEC_DYN | BI_BRAND_NOTE
+	.flags		= BI_CAN_EXEC_DYN | BI_BRAND_NOTE,
+	.header_supported= elf32_arm_abi_supported,
 };
 
 SYSINIT(oelf32, SI_SUB_EXEC, SI_ORDER_ANY,
 	(sysinit_cfunc_t) elf32_insert_brand_entry,
 	&freebsd_brand_oinfo);
 
+static boolean_t
+elf32_arm_abi_supported(struct image_params *imgp)
+{
+	const Elf_Ehdr *hdr = (const Elf_Ehdr *)imgp->image_header;
+
+#ifdef __ARM_EABI__
+	/*
+	 * When configured for EABI, FreeBSD supports EABI vesions 4 and 5.
+	 */
+	if (EF_ARM_EABI_VERSION(hdr->e_flags) < EF_ARM_EABI_FREEBSD_MIN) {
+		if (bootverbose)
+			uprintf("Attempting to execute non EABI binary (rev %d) image %s",
+			    EF_ARM_EABI_VERSION(hdr->e_flags), imgp->args->fname);
+		return (FALSE);
+	}
+#else
+	/*
+	 * When configured for OABI, that's all we do, so reject EABI binaries.
+	 */
+	if (EF_ARM_EABI_VERSION(hdr->e_flags) != EF_ARM_EABI_VERSION_UNKNOWN) {
+		if (bootverbose)
+			uprintf("Attempting to execute EABI binary (rev %d) image %s",
+			    EF_ARM_EABI_VERSION(hdr->e_flags), imgp->args->fname);
+		return (FALSE);
+	}
+#endif
+	return (TRUE);
+}
 
 void
 elf32_dump_thread(struct thread *td __unused, void *dst __unused,

sys/arm/include/elf.h

@@ -103,6 +103,12 @@ __ElfType(Auxinfo);
 #define	ELF_TARG_MACH	EM_ARM
 #define	ELF_TARG_VER	1
 
+/* Defines specific for arm headers */
+#define EF_ARM_EABIMASK      0xff000000
+#define EF_ARM_EABI_VERSION(x) (((x) & EF_ARM_EABIMASK) >> 24)
+#define EF_ARM_EABI_VERSION_UNKNOWN 0
+#define EF_ARM_EABI_FREEBSD_MIN 4
+
 /*
  * Magic number for the elf trampoline, chosen wisely to be an immediate
  * value.

sys/kern/imgact_elf.c

@@ -294,6 +294,19 @@ __elfN(get_brandinfo)(struct image_params *imgp, const char *interp,
 			return (bi);
 	}
 
+	/* No known brand, see if the header is recognized by any brand */
+	for (i = 0; i < MAX_BRANDS; i++) {
+		bi = elf_brand_list[i];
+		if (bi == NULL || bi->flags & BI_BRAND_NOTE_MANDATORY ||
+		    bi->header_supported == NULL)
+			continue;
+		if (hdr->e_machine == bi->machine) {
+			ret = bi->header_supported(imgp);
+			if (ret)
+				return (bi);
+		}
+	}
+
 	/* Lacking a known brand, search for a recognized interpreter. */
 	if (interp != NULL) {
 		for (i = 0; i < MAX_BRANDS; i++) {

sys/sys/imgact_elf.h

@@ -74,6 +74,7 @@ typedef struct {
 	const char *interp_newpath;
 	int flags;
 	Elf_Brandnote *brand_note;
+	boolean_t	(*header_supported)(struct image_params *);
 #define	BI_CAN_EXEC_DYN		0x0001
 #define	BI_BRAND_NOTE		0x0002	/* May have note.ABI-tag section. */
 #define	BI_BRAND_NOTE_MANDATORY	0x0004	/* Must have note.ABI-tag section. */