Re: [Haskell-cafe] createProcess running non-existent programs

13 Aug 2012

      This isn't that hard - a pipe shouldn't be needed anymore.  Just require a
post-2003 glibc.

fexecve is a system call in most BSDs.  It is also implemented in glibc
using a /proc hack.

http://www.kernel.org/doc/man-pages/online/pages/man3/fexecve.3.html

Apparently, there are proposals/RFCs to get a system called execveat into
the linux kernel which makes this work properly without /proc.

http://www.gossamer-threads.com/lists/linux/kernel/1574831

Alexander

On 13 August 2012 11:23, David Feuer  wrote:
...
In Unix, at least, "check, then act" is generally considered unwise:
1. Something can go wrong between checking and acting.
2. You might not be checking the right thing(s).  In this case, the fact
that the file exists is not useful if you don't have permission to execute
it. You may not be able to determine whether you have the appropriate
permissions without fairly deep manipulation of ACLs.
3. Even if the OS has the info at hand, making the system call(s)
necessary to get it is not free. Various non-free things happen every time
control passes between user-space and kernel-space.
On Aug 13, 2012 4:17 AM, "Andrew Cowie" 
wrote:
...
On Sun, 2012-08-12 at 23:18 -0700, Evan Laforge wrote:
...
Yes, I ran into the same thing a while back.  The problem is that the
subprocess has already been forked off before it runs exec() and finds
out the file doesn't exist.
Given how astonishingly common it is to pass an invalid executable name
and/or path, wouldn't it be worth doing a quick probe to see if the file
exists before createProcess actually forks?
[It's not like the effort the OS is going to do for the stat is going to
be thrown away; whether that call pulls it up off of disk or the one
after the fork that exec will do doesn't matter]
AfC
Sydney
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Re: [Haskell-cafe] createProcess running non-existent programs

Alexander Kjeldaas