Don’t trust st_size when scanning file head+tail

* src/fileio.c (Finsert_file_contents): Do not look at st_size
when scanning the file’s head and tail for a coding system comment.
Instead, just use read and SEEK_END and don’t worry if the latter fails.
st_size and lseek might not work in a /proc file system.

1 files changed, 12 insertions, 15 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 82ee22d2187..a3a49d179c4 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4271,24 +4271,21 @@ by calling `format-decode', which see.  */)
             {
               /* Find a coding system specified in the heading two
                  lines or in the tailing several lines of the file.
-                 We assume that the 1K-byte and 3K-byte for heading
+                 Assume that the 1 KiB and 3 KiB for heading
                  and tailing respectively are sufficient for this
-                 purpose.  */
-              int nread;
-
-              if (st.st_size <= (1024 * 4))
-                nread = emacs_fd_read (fd, read_buf, 1024 * 4);
-              else
+                 purpose.  Because the file may be in /proc,
+                 do not use st_size or report any SEEK_END failure.  */
+              ptrdiff_t nread = emacs_fd_read (fd, read_buf, 4 * 1024);
+              if (nread == 4 * 1024)
                 {
-                  nread = emacs_fd_read (fd, read_buf, 1024);
-                  if (nread == 1024)
+                  off_t tailoff = emacs_fd_lseek (fd, - 3 * 1024, SEEK_END);
+                  if (tailoff < 0)
+                    seekable = false;
+                  else if (1024 < tailoff)
                     {
-                      int ntail;
-                      if (emacs_fd_lseek (fd, st.st_size - 1024 * 3, SEEK_CUR) < 0)
-                        report_file_error ("Setting file position",
-                                           orig_filename);
-                      ntail = emacs_fd_read (fd, read_buf + nread, 1024 * 3);
-                      nread = ntail < 0 ? ntail : nread + ntail;
+                      ptrdiff_t ntail = emacs_fd_read (fd, read_buf + 1024,
+                                                       3 * 1024);
+                      nread = ntail < 0 ? ntail : 1024 + ntail;
                     }
                 }