[PATCH v8 2/7] Cygwin: pty: Add workaround for handling of backspace when pcon enabled
Takashi Yano
takashi.yano@nifty.ne.jp
Sat Mar 28 11:31:09 GMT 2026
On Sat, 28 Mar 2026 19:55:46 +0900
Takashi Yano wrote:
> In Windows 11, pseudo console has an undesired key conversion that
> the Ctrl-H is translated into Ctrl-Backspace (not Backspace).
> The reverse VT input path in conhost's `_DoControlCharacter()` maps
> the byte 0x08 to a Ctrl+Backspace key event (VK_BACK with
> LEFT_CTRL_PRESSED and character 0x7F). This was introduced in PR #3935
> (Jan 2020) to make Ctrl+Backspace delete whole words. In September
> 2022, PR #13894 rewrote the forward path to properly implement DECBKM
> (Backarrow Key Mode), but the reverse path was never updated to match,
> breaking the roundtrip.
>
> Due to this behaviour, inrec_eq() in cons_master_thread() fails to
> compare backspace/Ctrl-H events in the input record sequence. This
> patch is a workaround for the issue that replaces Ctrl-H with backspace
> (0x7f), which will be translated into Ctrl-H in pseudo console.
>
> Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
> Reviewed-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
> ---
> winsup/cygwin/fhandler/console.cc | 12 +++-
> winsup/cygwin/fhandler/pty.cc | 74 ++++++++++++++++++++++---
> winsup/cygwin/local_includes/fhandler.h | 2 +
> 3 files changed, 78 insertions(+), 10 deletions(-)
>
> diff --git a/winsup/cygwin/fhandler/console.cc b/winsup/cygwin/fhandler/console.cc
> index 2f46bbc6c..2a52ba575 100644
> --- a/winsup/cygwin/fhandler/console.cc
> +++ b/winsup/cygwin/fhandler/console.cc
> @@ -318,6 +318,16 @@ inrec_eq (const INPUT_RECORD *a, const INPUT_RECORD *b, DWORD n)
> written event. Therefore they are ignored. */
> const KEY_EVENT_RECORD *ak = &a[i].Event.KeyEvent;
> const KEY_EVENT_RECORD *bk = &b[i].Event.KeyEvent;
> + WCHAR c1 = ak->uChar.UnicodeChar;
> + WCHAR c2 = bk->uChar.UnicodeChar;
> + if (inside_pcon)
> + {
> + /* Workaround for pseudo console in Windows 11 */
> + if (c1 == 8) /* Ctrl-H */
> + c1 = 127; /* Backspace */
> + if (c2 == 8) /* Ctrl-H */
> + c2 = 127; /* Backspace */
> + }
> /* On Windows 11, conhost normalizes wRepeatCount from 0 to 1
> on readback. Treat them as equivalent for comparison. */
> WORD r1 = ak->wRepeatCount;
> @@ -327,7 +337,7 @@ inrec_eq (const INPUT_RECORD *a, const INPUT_RECORD *b, DWORD n)
> if (r2 == 0)
> r2 = 1;
> if (ak->bKeyDown != bk->bKeyDown
> - || ak->uChar.UnicodeChar != bk->uChar.UnicodeChar
> + || c1 != c2
> || r1 != r2)
> return false;
> }
> diff --git a/winsup/cygwin/fhandler/pty.cc b/winsup/cygwin/fhandler/pty.cc
> index fbc6152e5..7a7987c04 100644
> --- a/winsup/cygwin/fhandler/pty.cc
> +++ b/winsup/cygwin/fhandler/pty.cc
> @@ -1950,7 +1950,8 @@ fhandler_pty_master::fhandler_pty_master (int unit, dev_t via)
> master_thread (NULL), from_master_nat (NULL), to_master_nat (NULL),
> from_slave_nat (NULL), to_slave_nat (NULL), echo_r (NULL), echo_w (NULL),
> dwProcessId (0), to_master (NULL), from_master (NULL),
> - master_fwd_thread (NULL)
> + master_fwd_thread (NULL), h_pcon_in_dupped (NULL),
> + nat_pipe_owner_pid_dupped (0)
> {
> dev_referred_via = via;
> if (unit >= 0)
> @@ -2131,6 +2132,10 @@ fhandler_pty_master::close (int flag)
> termios_printf ("error closing from_master %p, %E", from_master);
> from_master = NULL;
>
> + if (h_pcon_in_dupped)
> + ForceCloseHandle (h_pcon_in_dupped);
> + h_pcon_in_dupped = NULL;
> +
> return 0;
> }
>
> @@ -2241,28 +2246,75 @@ fhandler_pty_master::write (const void *ptr, size_t len)
> { /* Reaches here when non-cygwin app is foreground and pseudo console
> is activated. */
> tmp_pathbuf tp;
> - char *buf = (char *) ptr;
> + char *buf = tp.c_get ();
> size_t nlen = len;
> if (get_ttyp ()->term_code_page != CP_UTF8)
> {
> static mbstate_t mbp;
> - buf = tp.c_get ();
> nlen = NT_MAX_PATH;
> convert_mb_str (CP_UTF8, buf, &nlen,
> get_ttyp ()->term_code_page, (const char *) ptr, len,
> &mbp);
> }
> + else
> + memcpy (buf, ptr, nlen);
>
> - for (size_t i = 0; i < nlen; i++)
> + if (get_ttyp ()->nat_pipe_owner_pid != nat_pipe_owner_pid_dupped
> + && !nat_pipe_owner_self (get_ttyp ()->nat_pipe_owner_pid))
> + {
> + if (h_pcon_in_dupped)
> + ForceCloseHandle (h_pcon_in_dupped);
> + h_pcon_in_dupped = NULL;
> + nat_pipe_owner_pid_dupped = 0;
> + HANDLE pcon_owner = OpenProcess (PROCESS_DUP_HANDLE, FALSE,
> + get_ttyp ()->nat_pipe_owner_pid);
> + if (pcon_owner)
> + {
> + DuplicateHandle (pcon_owner, get_ttyp ()->h_pcon_in,
> + GetCurrentProcess (), &h_pcon_in_dupped,
> + 0, FALSE, DUPLICATE_SAME_ACCESS);
> + nat_pipe_owner_pid_dupped = get_ttyp ()->nat_pipe_owner_pid;
> + CloseHandle(pcon_owner);
> + }
> + }
> + else
> + {
> + h_pcon_in_dupped = get_ttyp ()->h_pcon_in;
> + nat_pipe_owner_pid_dupped = get_ttyp ()->nat_pipe_owner_pid;
> + }
Oops! This is wrong.
Should be:
+ if (get_ttyp ()->nat_pipe_owner_pid != nat_pipe_owner_pid_dupped)
+ {
+ if (!nat_pipe_owner_self (get_ttyp ()->nat_pipe_owner_pid))
+ {
+ if (h_pcon_in_dupped)
+ ForceCloseHandle (h_pcon_in_dupped);
+ h_pcon_in_dupped = NULL;
+ nat_pipe_owner_pid_dupped = 0;
+ HANDLE pcon_owner = OpenProcess (PROCESS_DUP_HANDLE, FALSE,
+ get_ttyp ()->nat_pipe_owner_pid);
+ if (pcon_owner)
+ {
+ DuplicateHandle (pcon_owner, get_ttyp ()->h_pcon_in,
+ GetCurrentProcess (), &h_pcon_in_dupped,
+ 0, FALSE, DUPLICATE_SAME_ACCESS);
+ nat_pipe_owner_pid_dupped = get_ttyp ()->nat_pipe_owner_pid;
+ CloseHandle(pcon_owner);
+ }
+ }
+ else
+ {
+ h_pcon_in_dupped = get_ttyp ()->h_pcon_in;
+ nat_pipe_owner_pid_dupped = get_ttyp ()->nat_pipe_owner_pid;
+ }
+ }
> +
> + /* Retrieve console mode */
> + DWORD cons_mode = ENABLE_VIRTUAL_TERMINAL_INPUT;
> + if (h_pcon_in_dupped && memchr (buf, '\010' /* Ctrl-H */, nlen))
> + {
> + if (!nat_pipe_owner_self (nat_pipe_owner_pid_dupped))
> + {
> + DWORD resume_pid =
> + attach_console_temporarily (nat_pipe_owner_pid_dupped);
> + GetConsoleMode (h_pcon_in_dupped, &cons_mode);
> + resume_from_temporarily_attach (resume_pid);
> + }
> + else
> + GetConsoleMode (h_pcon_in_dupped, &cons_mode);
> + }
> +
> + len = nlen;
> + for (size_t i = 0, j = 0; i < len; i++)
> {
> process_sig_state r = process_sigs (buf[i], get_ttyp (), this);
> - if (r == done_with_debugger)
> + if (r != done_with_debugger)
> {
> - for (size_t j = i; j < nlen - 1; j++)
> - buf[j] = buf[j + 1];
> - nlen--;
> - i--;
> + char c = buf[i];
> + /* Workaround for pseudo console in Windows 11 */
> + if (!(cons_mode & ENABLE_VIRTUAL_TERMINAL_INPUT))
> + /* Undesired backspace conversion in pseudo console does
> + not happen if ENABLE_VIRTUAL_TERMINAL_INPUT is set. */
> + if (c == '\010') /* Ctrl-H */
> + c = '\177'; /* Backspace */
> + buf[j++] = c;
> }
> + else
> + nlen--;
> }
>
> DWORD n;
> @@ -3998,6 +4050,10 @@ fhandler_pty_slave::transfer_input (tty::xfer_dir dir, HANDLE from, tty *ttyp,
> if (r[i].EventType == KEY_EVENT && r[i].Event.KeyEvent.bKeyDown)
> {
> DWORD ctrl_key_state = r[i].Event.KeyEvent.dwControlKeyState;
> + if (r[i].Event.KeyEvent.uChar.AsciiChar == '\010' /* Ctrl-H */
> + && !(ctrl_key_state & ALT_PRESSED))
> + /* Workaround for pseudo console in Windows 11 */
> + r[i].Event.KeyEvent.uChar.AsciiChar = '\177'; /* Backspace */
> if (r[i].Event.KeyEvent.uChar.AsciiChar)
> {
> if ((ctrl_key_state & ALT_PRESSED)
> diff --git a/winsup/cygwin/local_includes/fhandler.h b/winsup/cygwin/local_includes/fhandler.h
> index 16f55b4f7..7ea04a26c 100644
> --- a/winsup/cygwin/local_includes/fhandler.h
> +++ b/winsup/cygwin/local_includes/fhandler.h
> @@ -2564,6 +2564,8 @@ private:
> HANDLE thread_param_copied_event;
> HANDLE helper_goodbye;
> HANDLE helper_h_process;
> + HANDLE h_pcon_in_dupped;
> + DWORD nat_pipe_owner_pid_dupped;
>
> public:
> HANDLE get_echo_handle () const { return echo_r; }
> --
> 2.51.0
>
--
Takashi Yano <takashi.yano@nifty.ne.jp>
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