[PATCH 1/4] slurpfile: use lseek() on character nodes instead of fstat() for file size

[Sebastian Andrzej Siewior]

From: Sebastian Andrzej Siewior <bigeasy at linutronix.de>

fstat() is used to determine the filesize before read() and
it requires a filesystem. That it means it can not be used
on character nodes. This makes it impossible to obtains the
kernel from a char node like mtd or more likely ubi.
We can't use this in every case because files in /proc don't
support lseek(). This is required by the powerpc part to read
some device-tree entries.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
---
 kexec/kexec.c |   22 ++++++++++++++++++++--
 1 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/kexec/kexec.c b/kexec/kexec.c
index 0616091..3c3f73e 100644
--- a/kexec/kexec.c
+++ b/kexec/kexec.c
@@ -467,7 +467,7 @@ char *slurp_file(const char *filename, off_t *r_size)
 {
 	int fd;
 	char *buf;
-	off_t size, progress;
+	off_t size, progress, err;
 	ssize_t result;
 	struct stat stats;
 	
@@ -486,7 +486,25 @@ char *slurp_file(const char *filename, off_t *r_size)
 		die("Cannot stat: %s: %s\n",
 			filename, strerror(errno));
 	}
-	size = stats.st_size;
+	/*
+	 * Seek in case the kernel is a character node like /dev/ubi0_0.
+	 * This does not work on regular files which live in /proc and
+	 * we need this for some /proc/device-tree entries
+	 */
+	if (S_ISCHR(stats.st_mode)) {
+
+		size = lseek(fd, 0, SEEK_END);
+		if (size < 0)
+			die("Can not seek file %s: %s\n", filename, strerror(errno));
+
+		err = lseek(fd, 0, SEEK_SET);
+		if (err < 0)
+			die("Can not seek to the begin of file %s: %s\n", filename,
+					strerror(errno));
+	} else {
+		size = stats.st_size;
+	}
+
 	*r_size = size;
 	buf = xmalloc(size);
 	progress = 0;
-- 
1.6.0.2