[PATCH V2] kexec, x86: fix kexec when?boot_params.hardware_subarch != 0

[Simon Horman]

On Thu, Mar 31, 2011 at 05:21:15PM +0000, Chris Leech wrote:
> Simon Horman <horms at ...> writes:
> 
> > 
> > On Tue, Mar 29, 2011 at 09:04:53AM +0000, WANG Cong wrote:
> > > On Mon, 28 Mar 2011 14:50:11 -0700, Chris Leech wrote:
> > > 
> > > > kexec needs to keep the subarch setting the same as the running kernel
> > > > in the boot parameters, or the kernel will die in early setup.  I ran
> > > > into this with X86_SUBARCH_MRST, but it should apply to CE4100 and any
> > > > future subarch that uses non-default early setup code.
> > > > 
> > > > This patch requires debugfs mounted at /sys/kernel/debug, as that's the
> > > > only way I know of to get at the running kernels boot_params. Without
> > > > debugfs mounted it falls back to the current behavior of assuming
> > > > subarch 0.
> > > > 
> > > ...
> > > >  
> > > > +#define BOOT_PARAMS_DBGFS	"/sys/kernel/debug/boot_params/data"
> > > 
> > > A minor issue here is that you are using a hard-coded debugfs path,
> > > debugfs can be also mounted to /debug too, so it is better that if we can 
> > > search the path dynamically here.
> > 
> > Do you think it would be better for the code to loop through
> > a few common alternatives for the path?
> > 
> 
> Not fully tested, but instead of looping through possible mount points maybe 
> look through mtab in search of debugfs by fs type.
> 
> Something like this?

This approach looks good to me.
Could you do some more testing?

> kexec, x86: search for debugfs mountpoint in /etc/mtab
> 
> From: Chris Leech <christopher.leech at linux.intel.com>
> 
> Signed-off-by: Chris Leech <christopher.leech at linux.intel.com>
> ---
>  kexec/arch/i386/x86-linux-setup.c |   35 +++++++++++++++++++++++++++++++++--
>  1 files changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/kexec/arch/i386/x86-linux-setup.c b/kexec/arch/i386/x86-linux-
> setup.c
> index 50b32be..0528cea 100644
> --- a/kexec/arch/i386/x86-linux-setup.c
> +++ b/kexec/arch/i386/x86-linux-setup.c
> @@ -30,6 +30,7 @@
>  #include <linux/fb.h>
>  #include <unistd.h>
>  #include <dirent.h>
> +#include <mntent.h>
>  #include <x86/x86-linux.h>
>  #include "../../kexec.h"
>  #include "kexec-x86.h"
> @@ -397,14 +398,44 @@ out:
>  		real_mode->eddbuf_entries);
>  }
>  
> -#define BOOT_PARAMS_DBGFS	"/sys/kernel/debug/boot_params/data"
> +/*
> + * This really only makes sense for virtual filesystems that are only expected
> + * to be mounted once (sysfs, debugsfs, proc), as it will return the first
> + * instance listed in mtab.
> + */
> +char *find_mnt_by_fsname(char *fsname)
> +{
> +	FILE *mtab;
> +	struct mntent *mnt;
> +	char *mntdir;
> +
> +	mtab = setmntent("/etc/mtab", "r");
> +	if (!mtab)
> +		return NULL;
> +	for(mnt = getmntent(mtab); mnt; mnt = getmntent(mtab)) {
> +		if (strcmp(mnt->mnt_fsname, fsname) == 0)
> +			break;
> +	}
> +	mntdir = mnt ? strdup(mnt->mnt_dir) : NULL;
> +	endmntent(mtab);
> +	return mntdir;
> +}
>  
>  void setup_subarch(struct x86_linux_param_header *real_mode)
>  {
>  	int data_file;
>  	const off_t offset = offsetof(typeof(*real_mode), hardware_subarch);
> +	char *debugfs_mnt;
> +	char filename[PATH_MAX];
> +
> +	debugfs_mnt = find_mnt_by_fsname("debugfs");
> +	if (!debugfs_mnt)
> +		return;
> +	snprintf(filename, PATH_MAX, "%s/%s", debugfs_mnt, "boot_params/data");

It might be worth bailing out on truncation here.
Perhaps with an error message?

> +	filename[PATH_MAX-1] = 0;
> +	free(debugfs_mnt);
>  
> -	data_file = open(BOOT_PARAMS_DBGFS, O_RDONLY);
> +	data_file = open(filename, O_RDONLY);
>  	if (data_file < 0)
>  		return;
>  	if (lseek(data_file, offset, SEEK_SET) < 0)