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Re: Extend faq.using to discuss fork failures


On 19/08/2011 10:35 AM, Corinna Vinschen wrote:
On Aug 19 09:43, Ryan Johnson wrote:
Hi all,

I propose to add an entry to cygwin's faq.using which covers fork
failures. Frankly, I'm surprised it wasn't there years ago... it's
certainly frequently-asked, and the answer is always the same. Right
now users have to trawl the archives to figure out what to do (or
more likely, just blindly spam the list and get told to rebase
and/or trawl the list archives).
If you convert the text into a patch against faq-using.xml and send
it to cygwin-patches, I'd take it.
Silly me... I should have realized that even the web site's content would be under revision control.

Also, what is the status of "the spawn family of calls provided by
Cygwin" [1]? There's nothing about it at the API page [2], and a
search though the user guide [3] comes up empty as well.
Have a look into the section called "Process Creation".  Granted, it's
not much.  These functions are practically foster children only.
I was quoting from the Process Creation section. I can't find anywhere else that mentions spawn at all.

Searching
/usr/include turns up only /usr/include/process.h, which contains
only the function declarations and a single comment -- "This file
comes with MSDOS and WIN32 systems" -- indicating that Windows, not
cygwin, provides the functions (which, incidentally, are deprecated
No, no, Cygwin provides these functions as well.
Does that mean Cygwin provides an independent implementation of the functions which should be used instead of the ones from Windows, or just that those functions are among the windows-native calls which cygwin makes available out of the box?

  Apart from that, the
process.h file is a problem since it duplicates the exec function
declarations which are given in sys/unistd.h.  I'll remove them.  If you
want to document them as special Cygwin functions, feel free to add a
spawn.sgml file to winsup/cygwin which can be included into the section
about Cygwin-specific functions, like, for instance, path.sgml or
security.sgml.
Whether that makes sense (and what gets written) would depend on the answer to the above, I think.

3. With Vista and later, use peflagsall to set the TS-aware bit on
all cygwin dlls (see /usr/share/doc/Cygwin/rebase-3.0.1.README,
reboot needed for changes to take effect). This exploits a side
effect of address space layout randomization which (ironically)
causes dlls to nearly always load at the same address.
I'm not sure I ever read about that.  On one hand, the TS-aware flag is
set by gcc 4.x automatically, on the other hand, the TS flag is only
relevant for actual Terminal Servers.
Do you mean the dynamicbase flag, maybe, as described by Chuck at one
point, years ago, on the cygwin-apps list?  Still, I doubt that this
flag has any positive effect, as far as I understand how it works.
Oops... up to now I always thought tsaware was the flag that affected ASLR.

Reading from this MSDN article [1] clarified things a bit for me. All dynamicbase-marked dlls are randomized (including all system dlls). Unmarked dlls, and those which load them, will not be randomized. So, rebasing dynamicbase dlls would seem to mean little or nothing, and we might be able to get away with not rebasing dynamicbase dlls at all (on Vista and later, of course). In particular, shipping dynamicbase dlls would greatly reduce the need to run rebaseall, because the newly-arrived/clobbered dlls would go right into Windows' ASLR bitmap. Further, dlls which rebaseall can't catch (like those created and dlopened dynamically) might also get a bit of a break. I guess that argues for changing gcc/binutils rather than running peflagsall, tho.

Also, we might want to consider *not* marking .exe dynamicbase, because "When a thread starts in a process linked with /DYNAMICBASE, Windows Vista and later moves the thread's stack to a random location." There's no way to turn off heap or mmap randomization, tho, AFAICT.

[1] http://msdn.microsoft.com/en-us/library/bb430720.aspx

Ryan


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