MIPS: ELF: Interpret the NAN2008 file header flag

Handle the EF_MIPS_NAN2008 ELF file header flag and refuse execution
where there is no support in the FPU for the NaN encoding mode requested
by a binary invoked.  Ensure that the setting of the bit in the binary
matches one in any intepreter used.  Set the thread's initial FCSR
contents according to the value of the EF_MIPS_NAN2008.

Set the values of the FCSR ABS2008 and NAN2008 bits both to the same
value if possible, to take the approach taken with existing FPU hardware
into account.  As of now all implementations have both bits hardwired to
the same value, that is both are fixed at 0 or both are fixed at 1, even
though the architecture allows for implementations where the amount of
control implemented with each of these two individual bits is
independent of each other.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Fortune <Matthew.Fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11479/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/elf.h

@@ -12,7 +12,6 @@
 #include <linux/fs.h>
 #include <uapi/linux/elf.h>
 
-#include <asm/cpu-info.h>
 #include <asm/current.h>
 
 /* ELF header e_flags defines. */
@@ -44,6 +43,7 @@
 #define EF_MIPS_OPTIONS_FIRST	0x00000080
 #define EF_MIPS_32BITMODE	0x00000100
 #define EF_MIPS_FP64		0x00000200
+#define EF_MIPS_NAN2008		0x00000400
 #define EF_MIPS_ABI		0x0000f000
 #define EF_MIPS_ARCH		0xf0000000
 
@@ -305,7 +305,7 @@ do {									\
 									\
 	current->thread.abi = &mips_abi;				\
 									\
-	current->thread.fpu.fcr31 = boot_cpu_data.fpu_csr31;		\
+	mips_set_personality_nan(state);				\
 } while (0)
 
 #endif /* CONFIG_32BIT */
@@ -367,7 +367,7 @@ do {									\
 	else								\
 		current->thread.abi = &mips_abi;			\
 									\
-	current->thread.fpu.fcr31 = boot_cpu_data.fpu_csr31;		\
+	mips_set_personality_nan(state);				\
 									\
 	p = personality(current->personality);				\
 	if (p != PER_LINUX32 && p != PER_LINUX)				\
@@ -432,6 +432,7 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm,
 				       int uses_interp);
 
 struct arch_elf_state {
+	int nan_2008;
 	int fp_abi;
 	int interp_fp_abi;
 	int overall_fp_mode;
@@ -440,6 +441,7 @@ struct arch_elf_state {
 #define MIPS_ABI_FP_UNKNOWN	(-1)	/* Unknown FP ABI (kernel internal) */
 
 #define INIT_ARCH_ELF_STATE {			\
+	.nan_2008 = -1,				\
 	.fp_abi = MIPS_ABI_FP_UNKNOWN,		\
 	.interp_fp_abi = MIPS_ABI_FP_UNKNOWN,	\
 	.overall_fp_mode = -1,			\
@@ -451,6 +453,7 @@ extern int arch_elf_pt_proc(void *ehdr, void *phdr, struct file *elf,
 extern int arch_check_elf(void *ehdr, bool has_interpreter, void *interp_ehdr,
 			  struct arch_elf_state *state);
 
+extern void mips_set_personality_nan(struct arch_elf_state *state);
 extern void mips_set_personality_fp(struct arch_elf_state *state);
 
 #endif /* _ASM_ELF_H */

arch/mips/kernel/elf.c

@@ -11,6 +11,8 @@
 #include <linux/elf.h>
 #include <linux/sched.h>
 
+#include <asm/cpu-info.h>
+
 /* FPU modes */
 enum {
 	FP_FRE,
@@ -135,6 +137,10 @@ int arch_check_elf(void *_ehdr, bool has_interpreter, void *_interp_ehdr,
 		struct elf32_hdr e32;
 		struct elf64_hdr e64;
 	} *ehdr = _ehdr;
+	union {
+		struct elf32_hdr e32;
+		struct elf64_hdr e64;
+	} *iehdr = _interp_ehdr;
 	struct mode_req prog_req, interp_req;
 	int fp_abi, interp_fp_abi, abi0, abi1, max_abi;
 	bool elf32;
@@ -143,6 +149,32 @@ int arch_check_elf(void *_ehdr, bool has_interpreter, void *_interp_ehdr,
 	elf32 = ehdr->e32.e_ident[EI_CLASS] == ELFCLASS32;
 	flags = elf32 ? ehdr->e32.e_flags : ehdr->e64.e_flags;
 
+	/*
+	 * Determine the NaN personality, reject the binary if no hardware
+	 * support.  Also ensure that any interpreter matches the executable.
+	 */
+	if (flags & EF_MIPS_NAN2008) {
+		if (cpu_has_nan_2008)
+			state->nan_2008 = 1;
+		else
+			return -ENOEXEC;
+	} else {
+		if (cpu_has_nan_legacy)
+			state->nan_2008 = 0;
+		else
+			return -ENOEXEC;
+	}
+	if (has_interpreter) {
+		bool ielf32;
+		u32 iflags;
+
+		ielf32 = iehdr->e32.e_ident[EI_CLASS] == ELFCLASS32;
+		iflags = ielf32 ? iehdr->e32.e_flags : iehdr->e64.e_flags;
+
+		if ((flags ^ iflags) & EF_MIPS_NAN2008)
+			return -ELIBBAD;
+	}
+
 	if (!config_enabled(CONFIG_MIPS_O32_FP64_SUPPORT))
 		return 0;
 
@@ -266,3 +298,27 @@ void mips_set_personality_fp(struct arch_elf_state *state)
 		BUG();
 	}
 }
+
+/*
+ * Select the IEEE 754 NaN encoding and ABS.fmt/NEG.fmt execution mode
+ * in FCSR according to the ELF NaN personality.
+ */
+void mips_set_personality_nan(struct arch_elf_state *state)
+{
+	struct cpuinfo_mips *c = &boot_cpu_data;
+	struct task_struct *t = current;
+
+	t->thread.fpu.fcr31 = c->fpu_csr31;
+	switch (state->nan_2008) {
+	case 0:
+		break;
+	case 1:
+		if (!(c->fpu_msk31 & FPU_CSR_NAN2008))
+			t->thread.fpu.fcr31 |= FPU_CSR_NAN2008;
+		if (!(c->fpu_msk31 & FPU_CSR_ABS2008))
+			t->thread.fpu.fcr31 |= FPU_CSR_ABS2008;
+		break;
+	default:
+		BUG();
+	}
+}