Re: RFC PATCH: Don't use /proc/self/maps to calculate size of initial thread stack

[Michael Hudson-Doyle <michael.hudson@canonical.com>]

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On Tue, 13 Sept 2022 at 21:53, Florian Weimer via Libc-alpha <
libc-alpha@sourceware.org> wrote:

> * Zack Weinberg via Libc-alpha:
>
> > When pthread_getattr_np is applied to the initial thread, it has to
> > figure out how big the initial thread's stack is.  Since the initial
> > thread's stack is lazily allocated and the kernel reuses that memory
> > region for the "information block" (argv, environ, etc) there are
> > several different ways one could define "the size of the initial
> > thread's stack"; for many years, the NPTL implementation has said that
> > the stack starts at __libc_stack_end, rounded in the opposite
> > direction from stack growth to the nearest page boundary, and extends
> > for getrlimit(RLIMIT_STACK).rlim_cur bytes, *minus the size of the
> > information block*, which is beyond __libc_stack_end.  The rationale
> > is that the resource limit is enforced against the entire memory area,
> > so if we don't subtract the size of the information block, then the
> > program will run out of stack a few pages before pthread_attr_getstack
> > says it will.
>
> Do we actually have to subtract the size of the information block?
> One could argue that this is just part of the arguments passed to main,
> so sort-of-but-not-quite part of main's stack frame.
>
> process_vm_readv seems quite likely to get blocked by seccomp filters.
>
> Maybe we can get the kernel to pass the end of the stack in the
> auxiliary vector?
>

I wondered the same when reading the first mail fwiw -- guessing what the
kernel did is always going to be more annoying than just having it tell
us...

Cheers,
mwh