fw_cfg: Splash image loader can overrun a stack variable, fix

read_splashfile() passes the address of an int variable as size_t *
parameter to g_file_get_contents(), with a cast to gag the compiler.

No problem on machines where sizeof(size_t) == sizeof(int).

Happens to work on my x86_64 box (64 bit little endian): the least
significant 32 bits of the file size end up in the right place
(caller's variable file_size), and the most significant 32 bits
clobber a place that gets assigned to before its next use (caller's
variable file_type).

I'd expect it to break on a 64 bit big-endian box.

Fix up the variable types and drop the problematic cast.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

hw/fw_cfg.c

@@ -54,7 +54,8 @@ struct FWCfgState {
 #define JPG_FILE 0
 #define BMP_FILE 1
 
-static char *read_splashfile(char *filename, int *file_sizep, int *file_typep)
+static char *read_splashfile(char *filename, size_t *file_sizep,
+                             int *file_typep)
 {
     GError *err = NULL;
     gboolean res;
@@ -63,7 +64,7 @@ static char *read_splashfile(char *filename, int *file_sizep, int *file_typep)
     unsigned int filehead = 0;
     int bmp_bpp;
 
-    res = g_file_get_contents(filename, &content, (gsize *)file_sizep, &err);
+    res = g_file_get_contents(filename, &content, file_sizep, &err);
     if (res == FALSE) {
         error_report("failed to read splash file '%s'", filename);
         g_error_free(err);
@@ -111,7 +112,7 @@ static void fw_cfg_bootsplash(FWCfgState *s)
     const char *boot_splash_filename = NULL;
     char *p;
     char *filename, *file_data;
-    int file_size;
+    size_t file_size;
     int file_type = -1;
     const char *temp;
 

include/sysemu/sysemu.h

@@ -122,7 +122,7 @@ extern int semihosting_enabled;
 extern int old_param;
 extern int boot_menu;
 extern uint8_t *boot_splash_filedata;
-extern int boot_splash_filedata_size;
+extern size_t boot_splash_filedata_size;
 extern uint8_t qemu_extra_params_fw[2];
 extern QEMUClock *rtc_clock;
 

vl.c

@@ -231,7 +231,7 @@ unsigned int nb_prom_envs = 0;
 const char *prom_envs[MAX_PROM_ENVS];
 int boot_menu;
 uint8_t *boot_splash_filedata;
-int boot_splash_filedata_size;
+size_t boot_splash_filedata_size;
 uint8_t qemu_extra_params_fw[2];
 
 typedef struct FWBootEntry FWBootEntry;