[PATCH] binfmt_elf: Reordered object list so compat_binfmt_elf optionally comes first.

[Ryan C . Gordon]

This will make it call register_binfmt() after normal binfmt_elf, inserting it
at the end of the list of binfmts.

Now the kernel will try the compatibility formats as a backup if the actual
system format rejects the binary. As almost all binaries loaded won't be
compatibility formats, this saves a few cycles for each process.

For scenarios where favoring compatibility binaries makes sense, this can
be chosen via the configuration, reversing the order.

Signed-off-by: Ryan C. Gordon <icculus@xxxxxxxxxxx>
---
 fs/Kconfig.binfmt |   14 ++++++++++++++
 fs/Makefile       |    7 +++++++
 2 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
index bb4cc5b..b296ab6 100644
--- a/fs/Kconfig.binfmt
+++ b/fs/Kconfig.binfmt
@@ -27,6 +27,20 @@ config COMPAT_BINFMT_ELF
 	bool
 	depends on COMPAT && BINFMT_ELF
 
+config FAVOR_COMPAT_BINFMT_ELF
+	bool "Favor compatibility ELF binaries"
+	depends on COMPAT_BINFMT_ELF
+	default n
+	---help---
+	  Favor "compatibility" ELF binaries. The system will work either way,
+	  but you can save a few cycles per process by choosing the type of
+	  binary you expect to be loading most of the time. This scenario 
+	  makes sense if you have a 64-bit kernel to manage 4+ gigabytes of
+	  physical RAM but want to run mostly 32-bit processes to conserve
+	  that RAM.
+
+	  If unsure, say N.
+
 config BINFMT_ELF_FDPIC
 	bool "Kernel support for FDPIC ELF binaries"
 	default y
diff --git a/fs/Makefile b/fs/Makefile
index af6d047..d835dba 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -40,8 +40,15 @@ obj-$(CONFIG_BINFMT_MISC)	+= binfmt_misc.o
 # binfmt_script is always there
 obj-y				+= binfmt_script.o
 
+# binfmts are registered in order listed here. First registered, first tried!
+ifeq ($(CONFIG_FAVOR_COMPAT_BINFMT_ELF),y)
+obj-$(CONFIG_COMPAT_BINFMT_ELF)	+= compat_binfmt_elf.o
+obj-$(CONFIG_BINFMT_ELF)	+= binfmt_elf.o
+else
 obj-$(CONFIG_BINFMT_ELF)	+= binfmt_elf.o
 obj-$(CONFIG_COMPAT_BINFMT_ELF)	+= compat_binfmt_elf.o
+endif
+
 obj-$(CONFIG_BINFMT_ELF_FDPIC)	+= binfmt_elf_fdpic.o
 obj-$(CONFIG_BINFMT_SOM)	+= binfmt_som.o
 obj-$(CONFIG_BINFMT_FLAT)	+= binfmt_flat.o
-- 
1.6.0.4

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