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[PATCH 2/2 v4] openpty: use TIOCGPTPEER to open slave side fd

• From: Christian Brauner <christian dot brauner at ubuntu dot com>
• To: libc-alpha at sourceware dot org,stgraber at stgraber dot org,serge at hallyn dot com,fweimer at redhat dot com,joseph at codesourcery dot com
• Cc: Christian Brauner <christian dot brauner at ubuntu dot com>
• Date: Tue, 29 Aug 2017 15:45:15 +0200
• Subject: [PATCH 2/2 v4] openpty: use TIOCGPTPEER to open slave side fd
• Authentication-results: sourceware.org; auth=none
• References: <a05d5603-c074-966d-f75e-cb268d0dc249@redhat.com> <20170829134515.9345-1-christian.brauner@ubuntu.com>

Newer kernels expose the ioctl TIOCGPTPEER [1] call to userspace which allows tosafely allocate a file descriptor for a pty slave based solely on the masterfile descriptor. This allows us to avoid path-based operations and makes thisfunction a lot safer in the face of devpts mounts in different mount namespaces.[1]:https://patchwork.kernel.org/patch/9760743/Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>---Changelog 2017-08-28:* Instead of #ifdefing the TIOCGPTPEER ioctl flag we now try the ioctl() first  and if it fails we fallback to path-based allocation of the slave fd. This  allows us retain backward compatibility with kernels that do not support this  ioctl call.* A note on the following codepath   if (name != NULL)     {       if (*buf == '\0')         if (pts_name (master, &buf, sizeof (_buf)))           goto fail;       strcpy (name, buf);     }  "buf" is guaranteed to be allocated in this case. If the pts_name() call above  failed we would have never reached this code path. If it has been called  succesfully it will either have handed us a valid buffer or "buf" will still  point to the static char array "_buf" which is initialized to 0.Changelog 2017-08-28:* Preserve #ifdef for TIOCGPTPEER since it needs to work on non-Linux distros  too.* Only intialize first byte of "_buf".Changelog 2017-08-29:* Adapt to unified error handling as suggested by Florian.--- ChangeLog       |  5 +++++ login/openpty.c | 30 ++++++++++++++++++++++++------ 2 files changed, 29 insertions(+), 6 deletions(-)diff --git a/ChangeLog b/ChangeLogindex bc5fb8e27f..30829e4c16 100644--- a/ChangeLog+++ b/ChangeLog@@ -1,3 +1,8 @@+2017-08-26  Christian Brauner  <christian.brauner@ubuntu.com>++* login/openpty.c (openpty): If defined, use the TIOCGPTPEER ioctl call+to allocate the slave pty file descriptor.+ 2017-08-26  Christian Brauner  <christian.brauner@ubuntu.com>  * login/openpty.c (openpty): Close slave pty file descriptor on error.diff --git a/login/openpty.c b/login/openpty.cindex a7b1ab5dde..766aa958ff 100644--- a/login/openpty.c+++ b/login/openpty.c@@ -94,6 +94,8 @@ openpty (int *amaster, int *aslave, char *name,   char *buf = _buf;   int master, ret = -1, slave = -1; +  *buf = '\0';+   master = getpt ();   if (master == -1)     return -1;@@ -104,12 +106,22 @@ openpty (int *amaster, int *aslave, char *name,   if (unlockpt (master))     goto on_error; -  if (pts_name (master, &buf, sizeof (_buf)))-    goto on_error;--  slave = open (buf, O_RDWR | O_NOCTTY);+#ifdef TIOCGPTPEER+  /* Try to allocate slave fd solely based on master fd first. */+  slave = ioctl (master, TIOCGPTPEER, O_RDWR | O_NOCTTY);+#endif   if (slave == -1)-    goto on_error;+    {+      /* Fallback to path-based slave fd allocation in case kernel doesn't+       * support TIOCGPTPEER.+       */+      if (pts_name (master, &buf, sizeof (_buf)))+        goto on_error;++      slave = open (buf, O_RDWR | O_NOCTTY);+      if (slave == -1)+        goto on_error;+    }    /* XXX Should we ignore errors here?  */   if (termp)@@ -122,7 +134,13 @@ openpty (int *amaster, int *aslave, char *name,   *amaster = master;   *aslave = slave;   if (name != NULL)-    strcpy (name, buf);+    {+      if (*buf == '\0')+        if (pts_name (master, &buf, sizeof (_buf)))+          goto on_error;++      strcpy (name, buf);+    }    ret = 0; -- 2.14.1

• References:
  • Re: [PATCH 1/2] openpty: close slave pty fd on error
    • From: Florian Weimer
  • [PATCH 1/2 v4] openpty: close slave pty fd on error
    • From: Christian Brauner