Usually, glibc inlines calls to non-cancellable versions of some
system calls, such as open_not_cancel.  The macro definitions are in
sysdeps/unix/sysv/linux/not-cancel.h.

This patch prevents those definitions from being inlined.  It moves
them into separate *.c files.

This is needed for building Plash's modified version of glibc.

Background: Plash is a secure, restricted execution environment that
provides functionality similar to chroot(), but more flexible and
lightweight.  It works partly by dynamically linking Linux executables
with a modified glibc.  This is not used for taking authority away
from a process, only for giving it back.  This approach is more
complete than using LD_PRELOADed libraries.  There's more information
on what Plash does at <http://plash.beasts.org>.

Plash builds its custom glibc by re-linking the object files produced
by the glibc build process.  It omits the object files for various
system calls and replaces them with its own code.  For this to work
completely, those system calls cannot be inlined.

The attached patch should not break the normal build of glibc.  I have
tested it for building the Debian glibc packages on i386, but not on
other architectures.

This patch isn't quite as essential for putting Plash into Debian as
the other one I filed in the BTS.


One complication in the patch is that Linuxthreads needs to refer to
some of the *_not_cancel functions.  I have left these as inlined,
conditionally, when used by Linuxthreads.  Usually Linuxthreads builds
its own versions of syscall object files (eg. ptw-close.os), but it
can't do this for close-not-cancel.os etc. because these are built
from C files, not from the assembler-based syscall generator.

The same approach is used for NPTL.  However, in this case, the
syscall generating code does generate non-cancelling versions of some
of the syscalls.  (This time they're called "close_nocancel"
etc. rather than "close_not_cancel".)  For those syscalls, NPTL uses,
for example, the ptw-close.os object file, while close-not-cancel.c is
replaced with an empty file.

Mark

burden for the Debian glibc maintainers, and it would have a
performance impact on all Debian applications on all architectures.

I fail to see why the core glibc package should be responsible for
building this rather mauled library.

What's Plash's CPU versions target?  For x86, you could probably do
this by:
 - replacing the dynamic linker instead of all of glibc
 - mapping a fake vsyscall page which checked the syscall number, and
   diverted to plash's code if appropriate
 - modifying the auxv vector to point at the modified vsyscall dso
   instead of the original
 - chaining to glibc's standard dynamic linker

Then you can do it with pristine binaries.  Should work on any
architecture which can indirect syscalls through a VDSO (at least ia64,
amd64, possibly soon ppc/ppc64).

I am a little dubious about the other Plash bug, but I'll
think about it.  It seems marginally within the purview of the
libc6-pic package and affects nothing else.

But it seems like it would be randomly crippled without this patch.

Daniel Jacobowitz <drow@false.org> wrote:

I don't think performance impact should be a real issue with this
patch.  The NPTL build of glibc doesn't inline these syscalls.  Most
system calls are not inlined in glibc anyway, and these *_not_cancel
calls don't seem to be used from functions that are performance
critical.  They seem to have been inlined as a convenient way of
getting code linked into libpthread.so for Linuxthreads, rather than
for performance.

I appreciate that it would be a maintenance burden.  I'll see if I can
get the patch accepted upstream.

I created a Plash package that built its own copy of the glibc source.
I posted an RFS on debian-mentors, and people complained about
duplicating glibc.  So I changed it to build from libc6-pic.

Interesting idea.  Has anyone used the vsyscall mechanism for
intercepting syscalls?

Having looked into this, one problem is that it won't work with the
"libc" and "nptl" builds of glibc that Debian does, because these use
"int $0x80" directly.  It would only work with the "i686" build.  So
this won't work with Linux 2.4 or with pre-686 processors.

Is your doubt about which package these files would go into, or about
these files going into a binary package at all?  ie. Would you prefer
a new libc6-blah package for putting these files in?
glibc functions that call *_not_cancel don't work.  It would be a
useful starting point.

As an alternative, I could build Plash from the NPTL build of glibc:
the relevant calls are already not inlined.  That would involve
changing the patch to put the NPTL object files into libc6-pic
instead.  But it would require Linux 2.6.

Mark

I would be amazed if upstream took it.  They are not generally tolerant
of this sort of limited-use-large-effect change.

Unlikely.  Most people need to intercept all syscalls, not just most.
It's not useful for that.

Correct.  It wouldn't anyway; Linux 2.4 did not have a vsyscall.  I
would have thought it would work with the NPTL build, but I didn't
check.

Definitely not a new package.

Huh?  Then why did your patch need to modify NPTL?

Daniel Jacobowitz <drow@false.org> wrote:

From my reading of the code, glibc will use a vsyscall entry point if
AT_SYSINFO is defined in the auxv, and this doesn't necessarily
require Linux 2.6.

NPTL inlines some but not all of the not-cancel.h calls.  I didn't
need them all un-inlined, but for completeness I un-inlined all of
them, in case I needed to replace these calls in the future.

Mark

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