[PATCH] kexec: determine size of block device

[Andreas Fenkart]

Hi Simon,

2015-10-16 3:30 GMT+02:00 Simon Horman <horms at verge.net.au>:
> On Fri, Oct 09, 2015 at 03:27:18PM +0200, Andreas Fenkart wrote:
>> starting 'kexec -l /dev/mmcblk0p1' fails since the size of
>> a block device can not be determined with stat
>
> Could you provide some motivation for kexec reading a kernel image
> from a block device as per the example above?

We plan to use it for our embedded system

We have an interim initramfs, that selects the kernel we want to boot
our real system. Something like a n+1 stage boot loader,
where n is the last stage of u-boot. The motivation is to do a safe
kernel upgrade on the production rootfs through package management.
The new kernel is dropped into a folder where the interim system will
selects the kernel with the highest priority. No reconfiguration of
u-boot/rescue system needed. In case no kernel has yet been installed,
we fall back to the factory default, which is installed in a raw flash
partition on eMMC without filesystem.

https://git.digitalstrom.org/bsp/dss-rescue-tools/blob/master/src/kernel-fallback-dss11-1gb-t1.config

>> Signed-off-by: Andreas Fenkart <andreas.fenkart at dev.digitalstrom.org>
>> ---
>>  kexec/kexec.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/kexec/kexec.c b/kexec/kexec.c
>> index ff024f3..8b8846e 100644
>> --- a/kexec/kexec.c
>> +++ b/kexec/kexec.c
>> @@ -26,7 +26,9 @@
>>  #include <stdlib.h>
>>  #include <errno.h>
>>  #include <limits.h>
>> +#include <sys/ioctl.h>
>>  #include <sys/mman.h>
>> +#include <sys/mount.h>
>>  #include <sys/types.h>
>>  #include <sys/stat.h>
>>  #include <sys/reboot.h>
>> @@ -554,6 +556,14 @@ char *slurp_file(const char *filename, off_t *r_size)
>>                       die("Can not seek to the begin of file %s: %s\n",
>>                                       filename, strerror(errno));
>>               buf = slurp_fd(fd, filename, size, &nread);
>> +    } else if (S_ISBLK(stats.st_mode)) {
>> +             /* taken from blockdev */
>
> Is "blockdev" Linux kernel code?

It is a util-linux tool, which is GPL of course. I added the comment
as a reference, since I couldn't find a man page for BLKGETSIZE64.
Since it's a straight forward ioctl call, there is probably no
copyright violation at all, and I could well remove it.

>
>> +             unsigned long long llu = -1;
>> +             err = ioctl(fd, BLKGETSIZE64, &llu);
>> +             if (err < 0)
>> +                     die("Can't retrieve size of block device %s: %s\n",
>> +                             filename, strerror(errno));
>> +             size = llu;
>>       } else {
>>               size = nread = stats.st_size;
>>               buf = mmap(NULL, size,
>> --
>> 2.5.1
>>
>>
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>>