make fseek detect and produce an error for invalid whence arguments

this is a POSIX requirement. we previously relied on the underlying fd
(or other backend) seek operation to produce the error, but since
linux lseek now supports other seek modes (SEEK_DATA and SEEK_HOLE)
which do not interact well with stdio buffering, this is insufficient.
instead, explicitly check whence before performing any operations.

diff --git a/src/stdio/fseek.c b/src/stdio/fseek.c
index 439308f..c07f7e9 100644 (file)
--- a/src/stdio/fseek.c
+++ b/src/stdio/fseek.c
@@ -1,7 +1,14 @@
 #include "stdio_impl.h"
+#include <errno.h>
 
 int __fseeko_unlocked(FILE *f, off_t off, int whence)
 {
+       /* Fail immediately for invalid whence argument. */
+       if (whence != SEEK_CUR && whence != SEEK_SET && whence != SEEK_END) {
+               errno = EINVAL;
+               return -1;
+       }
+
        /* Adjust relative offset for unread data in buffer, if any. */
        if (whence == SEEK_CUR && f->rend) off -= f->rend - f->rpos;