[PATCH v2 6/8] arm: allow passing an ELF64 header to elf_check_arch()

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Wed, May 05, 2010 at 09:54:18AM +0300, Mika Westerberg wrote:
> This is needed to shut following compiler warning when CONFIG_PROC_VMCORE is
> enabled:
> 
> fs/proc/vmcore.c: In function 'parse_crash_elf64_headers':
> fs/proc/vmcore.c:500: warning: passing argument 1 of 'elf_check_arch' from
> incompatible pointer type
> 
> ELF32 and ELF64 headers have common fields of same size (namely e_ident and
> e_machine) which are checked in arm_elf_check_arch().

This patch is bogus - and shows the dangers of throwing casts into C code
to shut up warnings without first analysing the code.

Our elf_check_arch() uses:
	e_machine
	e_entry
	e_flags
thusly:
        if (x->e_machine != EM_ARM)
        if (x->e_entry & 1) {
        } else if (x->e_entry & 3)
        eflags = x->e_flags;

Now, the Elf32 header looks like this:

typedef struct elf32_hdr{
  unsigned char e_ident[EI_NIDENT];	/* 0x00 - 0x0F */
  Elf32_Half    e_type;			/* 0x10 - 0x11 */
  Elf32_Half    e_machine;		/* 0x12 - 0x13 */
  Elf32_Word    e_version;		/* 0x14 - 0x17 */
  Elf32_Addr    e_entry;		/* 0x18 - 0x1b */
  Elf32_Off     e_phoff;		/* 0x1c - 0x1f */
  Elf32_Off     e_shoff;		/* 0x20 - 0x23 */
  Elf32_Word    e_flags;		/* 0x24 - 0x27 */

and Elf64 header:

typedef struct elf64_hdr {
  unsigned char e_ident[EI_NIDENT];	/* 0x00 - 0x0F */
  Elf64_Half e_type;			/* 0x10 - 0x11 */
  Elf64_Half e_machine;			/* 0x12 - 0x13 */
  Elf64_Word e_version;			/* 0x14 - 0x17 */
  Elf64_Addr e_entry;			/* 0x18 - 0x1f */
  Elf64_Off e_phoff;			/* 0x20 - 0x27 */
  Elf64_Off e_shoff;			/* 0x28 - 0x2f */
  Elf64_Word e_flags;			/* 0x30 - 0x33 */

Notice that e_entry and e_flags are different sizes and/or different
offsets, so ARMs elf_check_arch can not work with elf64 headers.  So
with an ELF64 header, accessing e_flags will result in actually
accessing the top half of the 64-bit e_phoff, and accessing 32-bit
e_entry will get us the lower half of the 64-bit e_entry.

Now, here's the question: why does this crashkernel stuff want to
parse a 64-bit ELF header on a 32-bit only platform where the crashing
kernel will never generate a 64-bit ELF core file?