From e91dd46e3f65517ed9dbc6e431a56112a403cb3e Mon Sep 17 00:00:00 2001
From: David Kalnischkies <david@kalnischkies.de>
Date: Fri, 28 Apr 2017 17:43:29 +0200
Subject: ident a CD without changing directory

---
 apt-pkg/contrib/cdromutl.cc | 46 ++++++++++++++++++++++-----------------------
 1 file changed, 23 insertions(+), 23 deletions(-)

diff --git a/apt-pkg/contrib/cdromutl.cc b/apt-pkg/contrib/cdromutl.cc
index 428ef0161..3628f6173 100644
--- a/apt-pkg/contrib/cdromutl.cc
+++ b/apt-pkg/contrib/cdromutl.cc
@@ -184,26 +184,32 @@ bool IdentCdrom(string CD,string &Res,unsigned int Version)
    MD5Summation Hash;
    bool writable_media = false;
 
+   int dirfd = open(CD.c_str(), O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+   if (dirfd == -1)
+      return _error->Errno("open",_("Unable to read %s"),CD.c_str());
+
    // if we are on a writable medium (like a usb-stick) that is just
    // used like a cdrom don't use "." as it will constantly change,
    // use .disk instead
-   if (access(CD.c_str(), W_OK) == 0 && DirectoryExists(CD+string("/.disk"))) 
+   if (faccessat(dirfd, ".", W_OK, 0) == 0)
    {
-      writable_media = true;
-      CD = CD.append("/.disk");
-      if (_config->FindB("Debug::aptcdrom",false) == true)
-         std::clog << "Found writable cdrom, using alternative path: " << CD
-                   << std::endl;
+      int diskfd = openat(dirfd, "./.disk", O_RDONLY | O_DIRECTORY | O_CLOEXEC, 0);
+      if (diskfd != -1)
+      {
+	 close(dirfd);
+	 dirfd = diskfd;
+	 writable_media = true;
+	 CD = CD.append("/.disk");
+	 if (_config->FindB("Debug::aptcdrom",false) == true)
+	    std::clog << "Found writable cdrom, using alternative path: " << CD
+	       << std::endl;
+      }
    }
 
-   string StartDir = SafeGetCWD();
-   if (chdir(CD.c_str()) != 0)
-      return _error->Errno("chdir",_("Unable to change to %s"),CD.c_str());
-   
-   DIR *D = opendir(".");
-   if (D == 0)
+   DIR * const D = fdopendir(dirfd);
+   if (D == nullptr)
       return _error->Errno("opendir",_("Unable to read %s"),CD.c_str());
-      
+
    /* Run over the directory, we assume that the reader order will never
       change as the media is read-only. In theory if the kernel did
       some sort of wacked caching this might not be true.. */
@@ -222,31 +228,24 @@ bool IdentCdrom(string CD,string &Res,unsigned int Version)
       else
       {
 	 struct stat Buf;
-	 if (stat(Dir->d_name,&Buf) != 0)
+	 if (fstatat(dirfd, Dir->d_name, &Buf, 0) != 0)
 	    continue;
 	 strprintf(S, "%lu", (unsigned long)Buf.st_mtime);
       }
 
       Hash.Add(S.c_str());
       Hash.Add(Dir->d_name);
-   };
-
-   if (chdir(StartDir.c_str()) != 0) {
-      _error->Errno("chdir",_("Unable to change to %s"),StartDir.c_str());
-      closedir(D);
-      return false;
    }
-   closedir(D);
 
    // Some stats from the fsys
    std::string S;
    if (_config->FindB("Debug::identcdrom",false) == false)
    {
       struct statvfs Buf;
-      if (statvfs(CD.c_str(),&Buf) != 0)
+      if (fstatvfs(dirfd, &Buf) != 0)
 	 return _error->Errno("statfs",_("Failed to stat the cdrom"));
 
-      // We use a kilobyte block size to advoid overflow
+      // We use a kilobyte block size to avoid overflow
       if (writable_media)
       {
          strprintf(S, "%lu", (unsigned long)(Buf.f_blocks*(Buf.f_bsize/1024)));
@@ -260,6 +259,7 @@ bool IdentCdrom(string CD,string &Res,unsigned int Version)
    else
       strprintf(S, "-%u.debug", Version);
 
+   closedir(D);
    Res = Hash.Result().Value() + S;
    return true;
 }
-- 
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