On Mon, May 10, 2010 at 01:20:36PM +0200, ext Russell King - ARM Linux wrote:
> 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.
Thanks for comments.
I believe that when passing ELF64 header, it fails in following checks:
/* Make sure it's an ARM executable */
if (x->e_ident[EI_CLASS] != ELF_CLASS)
return 0;
if (x->e_machine != EM_ARM)
return 0;
ELF_CLASS is defined in arch/arm/include/asm/elf.h:
#define ELF_CLASS ELFCLASS32
So if class is different than ELFCLASS32 it returns 0 and never even try to
access other fields, right?
> 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?
I really don't know but fs/proc/vmcore.c is coded in such way that it supports
both types of ELF headers. It however, passes the header to elf_check_arch()
which in our case should fail if it is something else than ELF32 header.
Thanks,
MW
