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

[Chris Leech]

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?

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");
+	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)