libinstaller: implement syslinux_already_installed

syslinux_already_installed function will be used in both extlinux and
syslinux for checking if the boot sector has either the string
"SYSLINUX" or "EXTLINUX" in the OEMID field.

Signed-off-by: Paulo Alcantara <pcacjr@gmail.com>

diff --git a/libinstaller/syslxcom.c b/libinstaller/syslxcom.c
index b176f6d..1de85aa 100644 (file)
--- a/libinstaller/syslxcom.c
+++ b/libinstaller/syslxcom.c
@@ -284,3 +284,16 @@ int sectmap(int fd, sector_t *sectors, int nsectors)
 
     return sectmap_fib(fd, sectors, nsectors);
 }
+
+/*
+ * SYSLINUX installs the string 'SYSLINUX' at offset 3 in the boot
+ * sector; this is consistent with FAT filesystems.  Earlier versions
+ * would install the string "EXTLINUX" instead, handle both.
+ */
+int syslinux_already_installed(int dev_fd)
+{
+    char buffer[8];
+
+    xpread(dev_fd, buffer, 8, 3);
+    return !memcmp(buffer, "SYSLINUX", 8) || !memcmp(buffer, "EXTLINUX", 8);
+}

diff --git a/libinstaller/syslxcom.h b/libinstaller/syslxcom.h
index 39ca09d..bf186ca 100644 (file)
--- a/libinstaller/syslxcom.h
+++ b/libinstaller/syslxcom.h
@@ -18,5 +18,6 @@ ssize_t xpwrite(int fd, const void *buf, size_t count, off_t offset);
 void clear_attributes(int fd);
 void set_attributes(int fd);
 int sectmap(int fd, sector_t *sectors, int nsectors);
+int syslinux_already_installed(int dev_fd);
 
 #endif