A General or Miscellaneous Binary Image Activator

[Stacey Son]

Hi All,

I have written a miscellaneous (or general) binary interpreter image activator to support cross building mips64 ports using user mode qemu.   The source code can be found at:

	http://people.freebsd.org/~sson/imgact_binmisc/imgact_binmisc.h
	http://people.freebsd.org/~sson/imgact_binmisc/imgact_binmisc.c

I just saw Nathan Whitehorn's posting 'LLVM Image Activator' on this same list and hope we are not duplicating work.  Like Nathan's this code invokes a user-level interpreter or emulator when it recognizes the magic bytes in a header of a binary.  Unlike Nathan's it is much more general purpose.   It does this by using a magic field that may be up to 'IBE_MAGIC_MAX'  (or 256) bytes long , an offset value in the binary, and, optionally, a bit mask field the same size as the magic field that can mask out and ignore any of the bits in the magic. Therefore, it is very flexible in supporting binary file types which I needed to figure out the arch for ELF files.

Of course, image activators have been a source for security issues.  If this or something like this is include in the kernel then I would recommend it be optional.  It has proven to be very useful for creating a mixed arch cross build environment.  This allows running things like the cross compiler/toolchain and /bin/sh natively on the host system which significantly helps performance.  Using this, and the user mode qemu port I have been working on, I have successfully cross built well over 9000 mips64 packages from ports using an old amd64 athlon (FYI, see https://wiki.freebsd.org/QemuUserModeHowTo for more information about user mode qemu).

It is configured from user space using sysctl(3).  For example, the following is for adding an activator that runs mips64 executables on an amd64 host using "user mode" qemu:

#include <sys/imgact_binmisc.h>

#define MIPS64_ELF_MAGIC                			"\x7f\x45\x4c\x46\x02\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x08"
#define MIPS64_ELF_MASK   "\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff"

ximgact_binmisc_entry_t xbe; 

xbe.xbe_version = IBE_VERSION;
xbe.xbe_command = IBC_ADD;
xbe.xbe_flags = IBF_ENABLED | IBF_USE_MASK;
strlcpy(xbe.xbe_name, "mips64elf", (IBE_NAME_MAX - 1));
strlcpy((xbe.xbe_interpreter, "/usr/local/bin/qemu-mips64", (MAXPATHLEN -1));
xbe.xbe_offset = 0;
xbe.xbe_size = sizeof(MIPS64_ELF_MAGIC) - 1;
memcpy(xbe.xbe_magic, MIPS64_ELF_MAGIC, xbe.xbe_size);
memcpy(xbe.xbe_mask, MIPS64_ELF_MASK, xbe.xbe_size);

 int ret = sysctlbyname("kern.binmisc", NULL, NULL, &xbe, sizeof(xbe));

To disable/enable/delete the above entry in the kernel:

xbe.xbe_version = IBE_VERSION;
xbe.xbe_command = IBC_DISABLE; // or IBC_ENABLE or IBC_DELETE
strlcpy(xbe.xbe_name, "mips64elf", (IBE_NAME_MAX - 1));

int ret = sysctlbyname("kern.binmisc", NULL, NULL, &xbe, sizeof(xbe));

To list all the interperter entries that are currently configured:

ximagact_binmisc_list_t xbl;
size_t xbe_size = sizeof(xbe);
size_t xbl_size = sizeof(xbl);

xbe.xbe_version = IBE_VERSION;
xbe.xbe_command = IBC_LIST;
sysctlbyname("kern.binmisc", &xbl, &xbl_size, &xbe, sizeof(xbe));

 for(int i = 0; i < xbl.xbl_count; i++) {
	
	xbe.xbe_version = IBE_VERSION;
 	xbe.xbe_command = IBC_LOOKUP;
	
	strlcpy(xbe.xbe_name, xbl.xbl_names[i], (IBE_NAME_MAX - 1));
	
	ret = sysctlbyname("kern.binmisc", &xbe, &xbe_size, &xbe, sizeof(xbe));
	
	if (!ret) {
		// 'xbe' contains an interperter entry from the kernel for	
		// the given 'xbl.xbl_name[i]' name.
	}
 }

 The sysctlbyname() calls above may return the following errors in addition
 to the one from sysctl(3):

 [EINVAL]  Invalid argment in the input ximgact_binmisc_entry_t structure.
 [EEXIST]  Interpreter entry for given name already exist in kernel list. 
 [ENOMEM]  Allocating memory in the kernel failed.
 [ENOENT]  Interpreter entry for given name is not found.
 [ENOSPC]  Attempted to exceed maximum number of entries (IBE_MAX_ENTRIES).

Of course, comments, questions, concerns, etc. are welcome.

Best Regards,

-stacey.
sson at FreeBSD.org