kernel: HTTPS through wget, half-close, and /proc/<pid>/fd

busybox wget hands TLS to ssl_client over a socketpair and then does
shutdown(SHUT_WR). That cut the AF_UNIX link both ways, so ssl_client's
reply was a SIGPIPE and every https:// download hung. SHUT_WR is now a
half-close: the peer reads EOF and can still answer. TCP sockets get
shutdown() at all (tcp_shutdown for the FIN); it used to do nothing.

- exec keeps ignored signals and the signal mask, as execve(2) says.
- /proc/<pid>/fd: a link per descriptor -- the path it was opened by,
  socket:[n], pipe:[n], anon_inode:[eventpoll]. /proc/self too. The
  global inode bit moves above pid << 8; at 0x10000 it was /proc/256.
- fstat on a socket or pipe answers S_IFSOCK / S_IFIFO instead of EINVAL.
- poll, select and epoll wake as soon as a pipe, socket or input device
  changes, not on the next tick.
- Pipes are 64 KB of lazily allocated pages, copied in blocks, with
  PIPE_BUF atomicity: 300 MB through `yes | head | wc` in 7 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/fs/exec.c

@@ -574,11 +574,15 @@ exec_elf(char *path, struct argsrc *av, struct argsrc *ev)
   // first delivered signal jump to a stale address in the new image.
   // mmap_next likewise belongs to the old address space; left alone it
   // could hand out mmap ranges overlapping the new image's layout.
+  // Ignored signals stay ignored, and the mask is inherited (execve(2)):
+  // neither points into the old image. `nohup` and every program that ignores
+  // SIGPIPE before running a helper depend on it.
   memset(myproc()->vma, 0, sizeof(myproc()->vma));
-  memset(myproc()->sig_handler, 0, sizeof(myproc()->sig_handler));
+  for (int i = 0; i < 32; i++)
+    if (myproc()->sig_handler[i] != 1)
+      myproc()->sig_handler[i] = 0;
   memset(myproc()->sig_restorer, 0, sizeof(myproc()->sig_restorer));
   myproc()->fs_base          = 0;
-  myproc()->signal_mask      = 0;
   myproc()->pending_signals  = 0;
   myproc()->in_signal        = 0;
   myproc()->sigsusp_active   = 0;