MIPS: Support for 64-bit FP with O32 binaries

CPUs implementing MIPS32 R2 may include a 64-bit FPU, just as MIPS64 CPUs
do. In order to preserve backwards compatibility a 64-bit FPU will act
like a 32-bit FPU (by accessing doubles from the least significant 32
bits of an even-odd pair of FP registers) when the Status.FR bit is
zero, again just like a mips64 CPU. The standard O32 ABI is defined
expecting a 32-bit FPU, however recent toolchains support use of a
64-bit FPU from an O32 MIPS32 executable. When an ELF executable is
built to use a 64-bit FPU a new flag (EF_MIPS_FP64) is set in the ELF
header.

With this patch the kernel will check the EF_MIPS_FP64 flag when
executing an O32 binary, and set Status.FR accordingly. The addition
of O32 64-bit FP support lessens the opportunity for optimisation in
the FPU emulator, so a CONFIG_MIPS_O32_FP64_SUPPORT Kconfig option is
introduced to allow this support to be disabled for those that don't
require it.

Inspired by an earlier patch by Leonid Yegoshin, but implemented more
cleanly & correctly.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Paul Burton <paul.burton@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/6154/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/mips/kernel/binfmt_elfo32.c b/arch/mips/kernel/binfmt_elfo32.c
index 202e581e6096..7faf5f2bee25 100644
--- a/arch/mips/kernel/binfmt_elfo32.c
+++ b/arch/mips/kernel/binfmt_elfo32.c
@@ -28,6 +28,18 @@ typedef double elf_fpreg_t;
 typedef elf_fpreg_t elf_fpregset_t[ELF_NFPREG];
 
 /*
+ * In order to be sure that we don't attempt to execute an O32 binary which
+ * requires 64 bit FP (FR=1) on a system which does not support it we refuse
+ * to execute any binary which has bits specified by the following macro set
+ * in its ELF header flags.
+ */
+#ifdef CONFIG_MIPS_O32_FP64_SUPPORT
+# define __MIPS_O32_FP64_MUST_BE_ZERO   0
+#else
+# define __MIPS_O32_FP64_MUST_BE_ZERO   EF_MIPS_FP64
+#endif
+
+/*
  * This is used to ensure we don't load something for the wrong architecture.
  */
 #define elf_check_arch(hdr)                                             \
@@ -44,6 +56,8 @@ typedef elf_fpreg_t elf_fpregset_t[ELF_NFPREG];
         if (((__h->e_flags & EF_MIPS_ABI) != 0) &&                      \
             ((__h->e_flags & EF_MIPS_ABI) != EF_MIPS_ABI_O32))          \
                 __res = 0;                                              \
+        if (__h->e_flags & __MIPS_O32_FP64_MUST_BE_ZERO)                \
+                __res = 0;                                              \
                                                                         \
         __res;                                                          \
 })