Cannot exec() program outside of /bin if PATH is unset

Christian Franke
Fri Sep 12 21:14:00 GMT 2014

Eric Blake wrote:
> On 09/12/2014 11:02 AM, Christian Franke wrote:
>> If PATH variable is unset or does not contain /bin or /usr/bin,
>> exec("/not_bin/program", ...) fails because cygwin DLLs could not be
>> loaded.
>> This affects postfix which cleans the environment to the bare minimum
>> for security reasons.
>> (fortunately there is an easy workaround, so this does not block postfix
>> ITP)
>> Testcase:
>> $ cat
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> // #include <windows.h>
>> int main()
>> {
>>    unsetenv("PATH");
> This is undefined behavior, per POSIX.  POSIX recommends that you always
> leave PATH defined to at least a bare minimum of the results of
> confstr(_CS_PATH, ...); it also states that implementations are free to
> do what they want (in this case, crash) if you don't follow the
> recommendation:
> "If PATH is unset or is set to null, the path search is
> implementation-defined."

There is no POSIX PATH search needed in my testcase:
   execl("/usr/sbin/alternatives", ...)

The alternatives.exe could be found. The required DLLs could not be loaded.
POSIX does not specify anything about the load path of shared libraries.
On Linux, LD_LIBRARY_PATH is completely separate from PATH.
On Windows, the DLL load path is connected to PATH.

>> Enabling the SetDllDirectory() Win32 call fixes the problem.
>> Would possibly make sense to add this call to cygwin1.dll.
> That said, just because POSIX has already given us the
> get-out-of-jail-free card doesn't mean that we can't be nice and improve
> cygwin1.dll to try and help broken programs that unset PATH.

Hmm... is postfix actually broken?
Unsetting PATH is IMO sane (from the POSIX POV) if all exec() calls use 
absolute path names.


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