ARM: a.out: remove partial a.out support

a.out support on ARM requires that argc, argv and envp are passed in r0-r2 respectively, which requires hacking load_aout_binary to prevent argc being clobbered by the return code. Whilst mainline kernels do set the registers up in start_thread, the aout loader has never carried the hack in mainline.

Initialising the registers in this way actually goes against the libc expectations for ELF binaries, where argc, argv and envp are passed on the stack, with r0 being used to hold a pointer to an exit function for cleaning up after the dynamic linker if required. If the pointer is NULL, then it is ignored. When execing an ELF binary, Linux currently zeroes r0, then sets it to argc and then finally clobbers it with the return value of the execve syscall, so we actually end up with:

	r0 = 0
	stack[0] = argc
	r1 = stack[1] = argv
	r2 = stack[2] = envp

libc treats r1 and r2 as undefined. The clobbering of r0 by sys_execve works for user-spawned threads, but when executing an ELF binary from a kernel thread (via call_usermodehelper), the execve is performed on the ret_from_fork path, which restores r0 from the saved pt_regs, resulting in argc being presented to the C library. This has horrible consequences when the application exits, since we have an exit function registered using argc, resulting in a jump to hyperspace.

This patch solves the problem by removing the partial a.out support from arch/arm/ altogether.

---
 arch/arm/Kconfig                  |  1 -
 arch/arm/include/asm/a.out-core.h | 45 ---------------------------------------
 arch/arm/include/asm/processor.h  |  4 ----
 arch/arm/include/uapi/asm/Kbuild  |  1 -
 arch/arm/include/uapi/asm/a.out.h | 34 -----------------------------
 5 files changed, 85 deletions(-)
 delete mode 100644 arch/arm/include/asm/a.out-core.h
 delete mode 100644 arch/arm/include/uapi/asm/a.out.h

index ba412e0..82f06982 100644
@@ -20,7 +20,6 @@ config ARM
 	select GENERIC_STRNCPY_FROM_USER
 	select GENERIC_STRNLEN_USER
 	select HARDIRQS_SW_RESEND
-	select HAVE_AOUT
 	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_SECCOMP_FILTER

index 06e7d50..413f387 100644
@@ -54,7 +54,6 @@ struct thread_struct {
 
 #define start_thread(regs,pc,sp)					\
 ({									\
-	unsigned long *stack = (unsigned long *)sp;			\
 	memset(regs->uregs, 0, sizeof(regs->uregs));			\
 	if (current->personality & ADDR_LIMIT_32BIT)			\
 		regs->ARM_cpsr = USR_MODE;				\

@@ -65,9 +64,6 @@ struct thread_struct {
 	regs->ARM_cpsr |= PSR_ENDSTATE;					\
 	regs->ARM_pc = pc & ~1;		/* pc */			\
 	regs->ARM_sp = sp;		/* sp */			\
-	regs->ARM_r2 = stack[2];	/* r2 (envp) */			\
-	regs->ARM_r1 = stack[1];	/* r1 (argv) */			\
-	regs->ARM_r0 = stack[0];	/* r0 (argc) */			\
 	nommu_start_thread(regs);					\
 })
 

index 47bcb2d..18d76fd 100644
@@ -1,7 +1,6 @@
 # UAPI Header export list
 include include/uapi/asm-generic/Kbuild.asm
 
-header-y += a.out.h
 header-y += byteorder.h
 header-y += fcntl.h
 header-y += hwcap.h

deleted file mode 100644
index 083894b..0000000
@@ -1,34 +0,0 @@
-#ifndef __ARM_A_OUT_H__
-#define __ARM_A_OUT_H__
-
-#include <linux/personality.h>
-#include <linux/types.h>
-
-struct exec
-{
-  __u32 a_info;		/* Use macros N_MAGIC, etc for access */
-  __u32 a_text;		/* length of text, in bytes */
-  __u32 a_data;		/* length of data, in bytes */
-  __u32 a_bss;		/* length of uninitialized data area for file, in bytes */
-  __u32 a_syms;		/* length of symbol table data in file, in bytes */
-  __u32 a_entry;	/* start address */
-  __u32 a_trsize;	/* length of relocation info for text, in bytes */
-  __u32 a_drsize;	/* length of relocation info for data, in bytes */
-};
-
-/*
- * This is always the same
- */
-#define N_TXTADDR(a)	(0x00008000)
-
-#define N_TRSIZE(a)	((a).a_trsize)
-#define N_DRSIZE(a)	((a).a_drsize)
-#define N_SYMSIZE(a)	((a).a_syms)
-
-#define M_ARM 103
-
-#ifndef LIBRARY_START_TEXT
-#define LIBRARY_START_TEXT	(0x00c00000)
-#endif
-
-#endif /* __A_OUT_GNU_H__ */

1.8.2.2

I don't think we can remove this quite as easily as this - this is a user visible API change as it's part of the userland headers, and will break anything in userspace which includes <a.out.h>

Hi Russell,

Does this actually matter? If we're removing aout support anyway, those programs will be broken by that. Looking at powerpc, sparc, sh and others it seems as though their a.out.h headers were removed a while ago despite being exported to userspace. I managed to get a 2.6.25 tree to generate headers confirming this.

Will