Re: [PATCH] posix_spawn: use a larger min stack for -fstack-check [BZ #21253]

[Mike Frysinger <vapier at gentoo dot org>]

On 16 Mar 2017 09:17, Florian Weimer wrote:
> On 03/16/2017 08:30 AM, Mike Frysinger wrote:
> > When glibc is built with -fstack-check, trying to use posix_spawn can
> > lead to segfaults due to gcc internally probing stack memory too far.
> > The new spawn API will allocate a minimum of 1 page, but the stack
> > checking logic might probe a couple of pages.  When it tries to walk
> > them, everything falls apart.
> >
> > The gcc internal docs [1] state the default interval checking is one
> > page.  Which means we need two pages (the current one, and the next
> > probed).  No target currently defines it larger.
> 
> GCC miscomputes the offsets in some cases, so I would not rely on this.
> 
> Would it be possible compile the functions involved without 
> -fstack-check instead?

i mentioned in the bug that it's not feasible: compiling this one file
doesn't help as it calls other glibc funcs which will have checking
enabled.  so we'd have to manually track the full call stack here and
disable it on all the files which is a fairly fragile/burdensome process.

> >    /* Add a slack area for child's stack.  */
> >    size_t argv_size = (argc * sizeof (void *)) + 512;
> > -  size_t stack_size = ALIGN_UP (argv_size, GLRO(dl_pagesize));
> > +  /* We need at least a few pages in case the compiler's stack checking is
> > +     enabled.  In some configs, it is known to use at least 24KiB.  */
> > +  size_t stack_size = ALIGN_UP (argv_size, 32 * 1024);
> 
> 64 KiB pages are common, so this reduces the stack size in many cases.

common where ?  are you talking about large pages ?

going by PAGE_SHIFT in the kernel, looks to me like very few
targets allow using 64 KiB at all, let alone being common.

alpha: 8 KiB
arc: kconfig allows 4, 8 (default), or 16 KiB
arm: 4 KiB
arm64: kconfig allows 4, 16, or 64 KiB (default depends on # of VMA bits)
avr32: 4 KiB
cris: 8 KiB
frv: 16 KiB
hexagon: kconfig allows 4 (default), 16, 64, 256 KiB, or 1 MiB
ia64: kconfig allows 4, 8, 16 (default), or 64 KiB
m32r: 4 KiB
m68k: 4 (non-coldfire) or 8 KiB (coldfire)
meta: kconfig allows 4 (default), 8, or 16 KiB
microblaze: kconfig allows 4 (default), 16, or 64 KiB
mips: kconfig allows 4 (default for most, but gets tricky), 8, 16, 32, or 64 KiB
mn10300: 4 KiB
nios2: 4 KiB
openrisc: 8 KiB
parisc: kconfig allows 4 (default), 16, or 64 KiB
powerpc: kconfig allows 4 (default), 16, 64, or 256 KiB
s390: 4 KiB
score: 4 KiB
sh: 4 KiB (for no-mmu, you have other choices)
sparc32: 4 KiB
sparc64: 8 KiB
tile: kconfig allows 4, 16, or 64 KiB (default)
unicore32: 4 KiB
x86: 4 KiB
xtensa: 4 KiB

i picked 32 KiB as a trade off of it being more than big enough and
being a nice multiple of 4, 8, and 16, while not overtly allocating
too much otherwise unused.
-mike

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