Re: [PATCH][V2] vmcore-dmesg: Determine correct machine pointer size

[Dave Young <dyoung@redhat.com>]

On 11/14/2012 12:38 AM, Vivek Goyal wrote:

> A 32bit arch can prepare ELF64 headers. For example for PAE case to
> preresent file offsets 64bit but data size at the offset still remains
> 32bit. If we just base our decision based on EI_CLASS, then we will try
> to read 64bit data from file and can run into various issues.
> 
> We ran into following issue when we tried to run vmcore-dmesg on a 32bit
> PAE system vmcore which had 64bit elf headers.
> 
> No program header covering vaddr 0xc0a6a688c0b89100found kexec bug?
> 
> Basically we try to read value of log_buf variable from address
> log_buf_vaddr. We read in 64bit value and then pass that value again
> to vaddr_to_offset() in an attempt to get to actual log_buf start
> and get error message.
> 
> So determine the machine pointer size based on ELF class and arch and read
> the bytes from file accordingly.
> 
> v2: Fixed the code as per suggestion from Eric.
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>


Ack

> ---
>  vmcore-dmesg/vmcore-dmesg.c |   21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> Index: kexec-tools/vmcore-dmesg/vmcore-dmesg.c
> ===================================================================
> --- kexec-tools.orig/vmcore-dmesg/vmcore-dmesg.c	2012-08-01 13:27:09.304878654 -0400
> +++ kexec-tools/vmcore-dmesg/vmcore-dmesg.c	2012-11-13 11:32:33.184358230 -0500
> @@ -89,6 +89,24 @@ static uint64_t vaddr_to_offset(uint64_t
>  	exit(30);
>  }
>  
> +static unsigned machine_pointer_bits(void)
> +{
> +	uint8_t bits = 0;
> +
> +	/* Default to the size of the elf class */
> +	switch(ehdr.e_ident[EI_CLASS]) {
> +	case ELFCLASS32:        bits = 32; break;
> +	case ELFCLASS64:        bits = 64; break;
> +	}
> +
> +	/* Report the architectures pointer size */
> +	switch(ehdr.e_machine) {
> +	case EM_386:            bits = 32; break;
> +	}
> +
> +        return bits;
> +}
> +
>  static void read_elf32(int fd)
>  {
>  	Elf32_Ehdr ehdr32;
> @@ -389,7 +407,8 @@ static uint64_t read_file_pointer(int fd
>  {
>  	uint64_t result;
>  	ssize_t ret;
> -	if (ehdr.e_ident[EI_CLASS] == ELFCLASS64) {
> +
> +	if (machine_pointer_bits() == 64) {
>  		uint64_t scratch;
>  		ret = pread(fd, &scratch, sizeof(scratch), addr);
>  		if (ret != sizeof(scratch)) {
> 
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-- 
Thanks
Dave

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