[PATCH v3 3/6] crashdump: setup general hotplug support

[Eric DeVolder]

To allow direct modification of the elfcorehdr by the kernel, in
response to CPU and memory hot un/plug and/or online/offline events,
the following conditions must occur:

 - the elfcorehdr buffer must be excluded from the purgatory
   checksum/digest, and
 - the elfcorehdr segment must be large enough, and
 - the kernel must be notified that it can modify the elfcorehdr

Excluding the elfcorehdr buffer from the digest occurs in patch
"crashdump: exclude elfcorehdr segment from digest for hotplug".
If this is not done, a change to the elfcorehdr will cause the
purgatory check at panic time to fail, and kdump capture kernel
does not start.

For hotplug, the size of the elfcorehdr segment is obtained from the
kernel via the /sys/kernel/crash_elforehdr_size node.

The KEXEC_UPDATE_ELFCOREHDR flag indicates to the kernel that it can
make direct modifications to the elfcorehdr.

Signed-off-by: Eric DeVolder <eric.devolder at oracle.com>
---
 kexec/kexec.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kexec/kexec.c b/kexec/kexec.c
index d790748..0207608 100644
--- a/kexec/kexec.c
+++ b/kexec/kexec.c
@@ -1631,6 +1631,24 @@ int main(int argc, char *argv[])
 		die("--load-live-update can only be used with xen\n");
 	}
 
+	/* NOTE: Xen KEXEC_LIVE_UPDATE and KEXEC_UPDATE_ELFCOREHDR collide */
+	if (do_hotplug) {
+		const char *ces = "/sys/kernel/crash_elfcorehdr_size";
+		char *buf, *endptr = NULL;
+		off_t nread = 0;
+		buf = slurp_file_len(ces, sizeof(buf)-1, &nread);
+		if (buf) {
+			if (buf[nread-1] == '\n')
+				buf[nread-1] = '\0';
+			elfcorehdrsz = strtoul(buf, &endptr, 0);
+		}
+		if (!elfcorehdrsz || (endptr && *endptr != '\0'))
+			die("Path %s does not exist, the kernel needs CONFIG_CRASH_HOTPLUG\n", ces);
+		dbgprintf("ELFCOREHDR_SIZE %lu\n", elfcorehdrsz);
+		/* Indicate to the kernel it is ok to modify the elfcorehdr */
+		kexec_flags |= KEXEC_UPDATE_ELFCOREHDR;
+	}
+
 	fileind = optind;
 	/* Reset getopt for the next pass; called in other source modules */
 	opterr = 1;
-- 
2.39.3