From fc6f690727aac2b5c9bd368f9b13188fb0ba36a7 Mon Sep 17 00:00:00 2001
From: user <user@clank>
Date: Thu, 16 Jul 2026 13:07:28 +0200
Subject: fs: reject open()/read() on a directory (EISDIR)

open() now checks the target's inode type: opening a directory as a file
returns $FE (EISDIR) instead of a valid fd, so 'cat docs' no longer streams the
raw directory block as bytes. cat reports 'is a directory'; wc/head/save treat
any invalid fd (>3) as an open failure. (ls is unaffected - it uses the
structural opendir/list path, not open/getb.)
---
 c/cat.c    |  3 ++-
 c/gbos.h   |  1 +
 c/head.c   |  4 ++--
 c/save.c   |  2 +-
 c/wc.c     |  4 ++--
 src/fs.asm | 10 ++++++++++
 6 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/c/cat.c b/c/cat.c
index dde58cd..790cb88 100644
--- a/c/cat.c
+++ b/c/cat.c
@@ -6,7 +6,8 @@ void main(void) {
     if (argc > 0) {
         unsigned char fd = open(argv[0], O_READ);
         int ch;
-        if (fd == NOFD) { puts("cat: no such file"); nl(); return; }
+        if (fd == EISDIR) { puts("cat: is a directory"); nl(); return; }
+        if (fd == NOFD)   { puts("cat: no such file"); nl(); return; }
         while ((ch = fgetc(fd)) >= 0) putc((char)ch);
         close(fd);
     } else {
diff --git a/c/gbos.h b/c/gbos.h
index c93414f..2a99c0e 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -34,6 +34,7 @@ void          poweroff(void);   /* clean emulator shutdown (saves) */
 #define O_READ  0
 #define O_WRITE 1
 #define NOFD    0xFF
+#define EISDIR  0xFE     /* open() failed: path is a directory */
 
 /* process control (c/libc.s) */
 unsigned char fork(void);
diff --git a/c/head.c b/c/head.c
index e8bf846..3298e54 100644
--- a/c/head.c
+++ b/c/head.c
@@ -16,10 +16,10 @@ void main(void) {
     }
     if (fname) {
         fd = open(fname, O_READ);
-        if (fd == NOFD) { puts("head: no such file"); nl(); return; }
+        if (fd > 3) { puts("head: no such file"); nl(); return; }
     }
     for (;;) {
-        if (fd == NOFD) { char c = readc(); if (c == EOF) break; ch = c; }
+        if (fd > 3) { char c = readc(); if (c == EOF) break; ch = c; }
         else { ch = fgetc(fd); if (ch < 0) break; }
         if (lines < n) putc((char)ch);
         if (ch == '\n') lines++;
diff --git a/c/save.c b/c/save.c
index dd2b8cd..1755fea 100644
--- a/c/save.c
+++ b/c/save.c
@@ -7,7 +7,7 @@ void main(void) {
     char c;
     if (argc < 1) { puts("usage: save <name>"); nl(); return; }
     fd = open(argv[0], O_WRITE);
-    if (fd == NOFD) { puts("save: cannot create"); nl(); return; }
+    if (fd > 3) { puts("save: cannot create"); nl(); return; }
     for (;;) {
         c = readc();
         if (c == EOF) break;
diff --git a/c/wc.c b/c/wc.c
index 424876d..4fa48ee 100644
--- a/c/wc.c
+++ b/c/wc.c
@@ -14,10 +14,10 @@ void main(void) {
     for (i = 0; i < argc; i++) if (argv[i][0] != '-') { fname = argv[i]; break; }
     if (fname) {
         fd = open(fname, O_READ);
-        if (fd == NOFD) { puts("wc: no such file"); nl(); return; }
+        if (fd > 3) { puts("wc: no such file"); nl(); return; }
     }
     for (;;) {
-        if (fd == NOFD) { char c = readc(); if (c == EOF) break; ch = c; }
+        if (fd > 3) { char c = readc(); if (c == EOF) break; ch = c; }
         else { ch = fgetc(fd); if (ch < 0) break; }
         chars++;
         if (ch == '\n') lines++;
diff --git a/src/fs.asm b/src/fs.asm
index d543b0f..a4ad77e 100644
--- a/src/fs.asm
+++ b/src/fs.asm
@@ -678,6 +678,13 @@ sys_open::
     ld [wFsDir], a
     call dir_find               ; A = target inode or 0
     ld [wFsInode], a
+    or a
+    jr z, .notdir               ; doesn't exist yet: no type to check
+    call inode_ptr
+    ld a, [hl]                  ; I_TYPE
+    cp IT_DIR
+    jr z, .isdir                ; can't open a directory as a file
+.notdir
     ld a, [wFsMode]
     or a
     jr nz, .write
@@ -730,6 +737,9 @@ sys_open::
     ld a, c
     cp OF_MAX
     jr c, .ofl
+.isdir
+    ld a, $FE                   ; EISDIR: is a directory
+    ret
 .fail
     ld a, $FF
     ret
-- 
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