[PATCH] fs/binfmt_elf_fdpic.c: provide NOMMU loader for regular ELF binaries

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From: Rich Felker <dalias@xxxxxxxx>

The ELF binary loader in binfmt_elf.c requires an MMU, making it
impossible to use regular ELF binaries on NOMMU archs. However, the
FDPIC ELF loader in binfmt_elf_fdpic.c is fully capable as a loader
for plain ELF, which requires constant displacements between LOAD
segments, since it already supports FDPIC ELF files flagged as needing
constant displacement.

This patch adjusts the FDPIC ELF loader to accept non-FDPIC ELF files
on NOMMU archs. They are treated identically to FDPIC ELF files with
the constant-displacement flag bit set, except for personality, which
must match the ABI of the program being loaded; the PER_LINUX_FDPIC
personality controls how the kernel interprets function pointers
passed to sigaction.

Files that do not set a stack size requirement explicitly are given a
default stack size (matching the amount of committed stack the normal
ELF loader for MMU archs would give them) rather than being rejected;
this is necessary because plain ELF files generally do not declare
stack requirements in theit program headers.

Only ET_DYN (PIE) format ELF files are supported, since loading at a
fixed virtual address is not possible on NOMMU.

Signed-off-by: Rich Felker <dalias@xxxxxxxx>

This patch was developed and tested on J2 (SH2-compatible) but should
be usable immediately on all archs where binfmt_elf_fdpic is
available. Moreover, by providing dummy definitions of the
elf_check_fdpic() and elf_check_const_displacement() macros for archs
which lack an FDPIC ABI, it should be possible to enable building of
binfmt_elf_fdpic on all other NOMMU archs and thereby give them ELF
binary support, but I have not yet tested this.

The motivation for using binfmt_elf_fdpic.c rather than adapting
binfmt_elf.c to NOMMU is that the former already has all the necessary
code to work properly on NOMMU and has already received widespread
real-world use and testing. I hope this is not controversial.

I'm not really happy with having to unset the FDPIC_FUNCPTRS
personality bit when loading non-FDPIC ELF. This bit should really
reset automatically on execve, since otherwise, executing non-ELF
binaries (e.g. bFLT) from an FDPIC process will leave the personality
in the wrong state and severely break signal handling. But that's a
separate, existing bug and I don't know the right place to fix it.

--- fs/binfmt_elf_fdpic.c.orig	2015-09-29 22:13:06.716412478 +0000
+++ fs/binfmt_elf_fdpic.c	2015-09-29 22:38:24.122986621 +0000
@@ -103,19 +103,36 @@
-static int is_elf_fdpic(struct elfhdr *hdr, struct file *file)
+static int is_elf(struct elfhdr *hdr, struct file *file)
 	if (memcmp(hdr->e_ident, ELFMAG, SELFMAG) != 0)
 		return 0;
 	if (hdr->e_type != ET_EXEC && hdr->e_type != ET_DYN)
 		return 0;
-	if (!elf_check_arch(hdr) || !elf_check_fdpic(hdr))
+	if (!elf_check_arch(hdr))
 		return 0;
 	if (!file->f_op->mmap)
 		return 0;
 	return 1;
+#ifndef elf_check_fdpic
+#define elf_check_fdpic(x) 0
+#ifndef elf_check_const_displacement
+#define elf_check_const_displacement(x) 0
+static int is_constdisp(struct elfhdr *hdr)
+	if (!elf_check_fdpic(hdr))
+		return 1;
+	if (elf_check_const_displacement(hdr))
+		return 1;
+	return 0;
  * read the program headers table into memory
@@ -191,8 +208,16 @@
 	/* check that this is a binary we know how to deal with */
 	retval = -ENOEXEC;
-	if (!is_elf_fdpic(&exec_params.hdr, bprm->file))
+	if (!is_elf(&exec_params.hdr, bprm->file))
+		goto error;
+#ifdef CONFIG_MMU
+	/* binfmt_elf handles non-fdpic elf except on nommu */
+	if (!elf_check_fdpic(&exec_params.hdr))
+		goto error;
+	if (exec_params.hdr.e_type != ET_DYN)
 		goto error;
 	/* read the program header table */
 	retval = elf_fdpic_fetch_phdrs(&exec_params, bprm->file);
@@ -269,13 +294,13 @@
-	if (elf_check_const_displacement(&exec_params.hdr))
+	if (is_constdisp(&exec_params.hdr))
 		exec_params.flags |= ELF_FDPIC_FLAG_CONSTDISP;
 	/* perform insanity checks on the interpreter */
 	if (interpreter_name) {
 		retval = -ELIBBAD;
-		if (!is_elf_fdpic(&interp_params.hdr, interpreter))
+		if (!is_elf(&interp_params.hdr, interpreter))
 			goto error;
 		interp_params.flags = ELF_FDPIC_FLAG_PRESENT;
@@ -306,9 +331,9 @@
 	retval = -ENOEXEC;
 	if (stack_size == 0)
-		goto error;
+		stack_size = 131072UL; /* same as exec.c's default commit */
-	if (elf_check_const_displacement(&interp_params.hdr))
+	if (is_constdisp(&interp_params.hdr))
 		interp_params.flags |= ELF_FDPIC_FLAG_CONSTDISP;
 	/* flush all traces of the currently running executable */
@@ -319,7 +344,10 @@
 	/* there's now no turning back... the old userspace image is dead,
 	 * defunct, deceased, etc.
-	set_personality(PER_LINUX_FDPIC);
+	if (elf_check_fdpic(&exec_params.hdr))
+		set_personality(PER_LINUX_FDPIC);
+	else
+		set_personality(PER_LINUX);
 	if (elf_read_implies_exec(&exec_params.hdr, executable_stack))
 		current->personality |= READ_IMPLIES_EXEC;
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