hush: fix/explain corner cases of redirection colliding with script fd

function                                             old     new   delta
save_fd_on_redirect                                  200     208      +8
run_pipe                                            1870    1873      +3
setup_redirects                                      321     322      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 12/0)               Total: 12 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/shell/hush.c b/shell/hush.c
index 7a1513c2489d61d5fca58f758ff6773ee8cc4a09..e3c6e2de997d2617e28473d7de6a7dff21181e3a 100644 (file)
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -7470,16 +7470,54 @@ static int save_fd_on_redirect(int fd, int avoid_fd, struct squirrel **sqp)
                return 1; /* "we closed fd" */
        }
 #endif
+       /* Are we called from setup_redirects(squirrel==NULL)
+        * in redirect in a [v]forked child?
+        */
+       if (sqp == NULL) {
+               /* No need to move script fds.
+                * For NOMMU case, it's actively wrong: we'd change ->fd
+                * fields in memory for the parent, but parent's fds
+                * aren't be moved, it would use wrong fd!
+                * Reproducer: "cmd 3>FILE" in script.
+                * If we would call move_HFILEs_on_redirect(), child would:
+                *  fcntl64(3, F_DUPFD_CLOEXEC, 10)   = 10
+                *  close(3)                          = 0
+                * and change ->fd to 10 if fd#3 is a script fd. WRONG.
+                */
+               //bb_error_msg("sqp == NULL: [v]forked child");
+               return 0;
+       }
+
        /* If this one of script's fds? */
        if (move_HFILEs_on_redirect(fd, avoid_fd))
                return 1; /* yes. "we closed fd" (actually moved it) */
 
-       /* Are we called from setup_redirects(squirrel==NULL)? Two cases:
-        * (1) Redirect in a forked child.
-        * (2) "exec 3>FILE".
+       /* Are we called for "exec 3>FILE"? Came through
+        * redirect_and_varexp_helper(squirrel=ERR_PTR) -> setup_redirects(ERR_PTR)
+        * This case used to fail for this script:
+        *  exec 3>FILE
+        *  echo Ok
+        *  ...100000 more lines...
+        *  echo Ok
+        * as follows:
+        *  read(3, "exec 3>FILE\necho Ok\necho Ok"..., 1024) = 1024
+        *  open("FILE", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 4
+        *  dup2(4, 3)                        = 3
+        *  ^^^^^^^^ oops, we lost fd#3 opened to our script!
+        *  close(4)                          = 0
+        *  write(1, "Ok\n", 3)               = 3
+        *  ...                               = 3
+        *  write(1, "Ok\n", 3)               = 3
+        *  read(3, 0x94fbc08, 1024)          = -1 EBADF (Bad file descriptor)
+        *  ^^^^^^^^ oops, wrong fd!!!
+        * With this case separate from sqp == NULL and *after* move_HFILEs,
+        * it now works:
         */
-       if (!sqp)
+       if (sqp == ERR_PTR) {
+               /* Don't preserve redirected fds: exec is _meant_ to change these */
+               //bb_error_msg("sqp == ERR_PTR: exec >FILE");
                return 0;
+       }
 
        /* Check whether it collides with any open fds (e.g. stdio), save fds as needed */
        *sqp = add_squirrel(*sqp, fd, avoid_fd);
@@ -7618,7 +7656,7 @@ static int setup_redirects(struct command *prog, struct squirrel **sqp)
                         */
                } else {
                        /* if newfd is a script fd or saved fd, simulate EBADF */
-                       if (internally_opened_fd(newfd, sqp ? *sqp : NULL)) {
+                       if (internally_opened_fd(newfd, sqp && sqp != ERR_PTR ? *sqp : NULL)) {
                                //errno = EBADF;
                                //bb_perror_msg_and_die("can't duplicate file descriptor");
                                newfd = -1; /* same effect as code above */
@@ -8633,8 +8671,8 @@ static int checkjobs_and_fg_shell(struct pipe *fg_pipe)
  * subshell:     ( list ) [&]
  */
 #if !ENABLE_HUSH_MODE_X
-#define redirect_and_varexp_helper(command, squirrel, argv_expanded) \
-       redirect_and_varexp_helper(command, squirrel)
+#define redirect_and_varexp_helper(command, sqp, argv_expanded) \
+       redirect_and_varexp_helper(command, sqp)
 #endif
 static int redirect_and_varexp_helper(
                struct command *command,
@@ -8651,10 +8689,6 @@ static int redirect_and_varexp_helper(
        /* this takes ownership of new_env[i] elements, and frees new_env: */
        set_vars_and_save_old(new_env);
 
-       /* setup_redirects acts on file descriptors, not FILEs.
-        * This is perfect for work that comes after exec().
-        * Is it really safe for inline use?  Experimentally,
-        * things seem to work. */
        return setup_redirects(command, sqp);
 }
 static NOINLINE int run_pipe(struct pipe *pi)
@@ -8855,7 +8889,10 @@ static NOINLINE int run_pipe(struct pipe *pi)
                                 */
                                enter_var_nest_level();
                                G.shadowed_vars_pp = &old_vars;
-                               rcode = redirect_and_varexp_helper(command, /*squirrel:*/ NULL, argv_expanded);
+                               rcode = redirect_and_varexp_helper(command,
+                                       /*squirrel:*/ ERR_PTR,
+                                       argv_expanded
+                               );
                                G.shadowed_vars_pp = sv_shadowed;
                                /* rcode=1 can be if redir file can't be opened */