don't drop privileges if _apt has not enough rights

Privilege dropping breaks download/source/changelog commands as they
require the _apt user to have write permissions in the current directory,
which is e.g. the case in /tmp, but not in /root, so we disable the
privilege dropping if we deal with such a directory based on idea and
code by Michael Vogt.

The alternative would be to download always to a temp directory and move
it then done, but this breaks partial file support. To resolve this, we
could move to one of our partial/ directories, but this would require a
lock which would block root from using two of these commands in
parallel. As both seems unacceptable we instead let the user choose what
to do: Either a directory is setupped for _apt, downloading as root is
accepted or – which is potentially even better – an unprivileged user is
used for the commands.

1 files changed, 14 insertions, 5 deletions

diff --git a/cmdline/apt-get.cc b/cmdline/apt-get.cc
index 8448638db..8c0c50f83 100644
--- a/cmdline/apt-get.cc
+++ b/cmdline/apt-get.cc
@@ -676,6 +676,9 @@ static bool DoDownload(CommandLine &CmdL)
       return true;
    }
 
+   // Disable drop-privs if "_apt" can not write to the target dir
+   CheckDropPrivsMustBeDisabled(Fetcher);
+
    if (_error->PendingError() == true || CheckAuth(Fetcher, false) == false)
       return false;
 
@@ -858,10 +861,6 @@ static bool DoSource(CommandLine &CmdL)
       }
    }
 
-   // check authentication status of the source as well
-   if (UntrustedList != "" && !AuthPrompt(UntrustedList, false))
-      return false;
-   
    // Display statistics
    unsigned long long FetchBytes = Fetcher.FetchNeeded();
    unsigned long long FetchPBytes = Fetcher.PartialPresent();
@@ -908,7 +907,7 @@ static bool DoSource(CommandLine &CmdL)
 	 ioprintf(cout,_("Fetch source %s\n"),Dsc[I].Package.c_str());
       return true;
    }
-   
+
    // Just print out the uris an exit if the --print-uris flag was used
    if (_config->FindB("APT::Get::Print-URIs") == true)
    {
@@ -919,6 +918,13 @@ static bool DoSource(CommandLine &CmdL)
       return true;
    }
 
+   // Disable drop-privs if "_apt" can not write to the target dir
+   CheckDropPrivsMustBeDisabled(Fetcher);
+
+   // check authentication status of the source as well
+   if (UntrustedList != "" && !AuthPrompt(UntrustedList, false))
+      return false;
+
    // Run it
    bool Failed = false;
    if (AcquireRun(Fetcher, 0, &Failed, NULL) == false || Failed == true)
@@ -1510,6 +1516,9 @@ static bool DownloadChangelog(CacheFile &CacheFile, pkgAcquire &Fetcher,
    // queue it
    new pkgAcqFile(&Fetcher, changelog_uri, "", 0, descr, Pkg.Name(), "ignored", targetfile);
 
+   // Disable drop-privs if "_apt" can not write to the target dir
+   CheckDropPrivsMustBeDisabled(Fetcher);
+
    // try downloading it, if that fails, try third-party-changelogs location
    // FIXME: Fetcher.Run() is "Continue" even if I get a 404?!?
    Fetcher.Run();