madvise failed: Invalid argument

madvise failed: Invalid argument

[Felix Lechner via Developers list for Guile, the GNU extensibility library]

Hi,

Would someone please help me understand the warning message

    madvise failed: Invalid argument

in my code?

In my newest project, I hope to offer users a more convenient way to
configure authentication tasks and partially reimplemented Linux-PAM in
GNU Guile. [1]

The online manual is the most convenient way to read about it. [2]

My code uses an ELF shared object called pam_guile.so to call Guile via
the interface described in the Tortoise tutorial.  [3] The warning
probably originates here:

    ret = madvise ((void *) lo, hi - lo, MADV_DONTNEED); [4]

From my reading, that call is meant to return stack space back to the
operating system.  According to the madvise(2) manual page [5] the
variable 'lo' must be page-aligned, but apparently isn't.

Why would the 'stack_bottom' not be page aligned, please?  Could it be
mitigated by alignment instructions in my ELF shared object?

Thank you for looking!

Kind regards
Felix

P.S. Please copy me explicitly.  I do not subscribe to your list.

[1] https://codeberg.org/lechner/guile-pam
[2] https://juix.org/guile-pam/
[3] https://www.gnu.org/software/guile/docs/guile-tut/tutorial.html
[4] https://git.savannah.gnu.org/cgit/guile.git/tree/libguile/vm.c#n651
[5] https://www.man7.org/linux/man-pages/man2/madvise.2.html

Re: madvise failed: Invalid argument

[Olivier Dion]

On Mon, 12 Aug 2024, Felix Lechner via "Developers list for Guile, the GNU extensibility library" <guile-devel@gnu.org> wrote:
> Hi,
[...]

> The warning probably originates here:
>
>     ret = madvise ((void *) lo, hi - lo, MADV_DONTNEED); [4]
>
> From my reading, that call is meant to return stack space back to the
> operating system.  According to the madvise(2) manual page [5] the
> variable 'lo' must be page-aligned, but apparently isn't.
>
> Why would the 'stack_bottom' not be page aligned, please?  Could it be
> mitigated by alignment instructions in my ELF shared object?

The argument passed to `madvise(2)' is page aligned correcly.  See the
round down with `lo &= ~(page_size - 1U); /* round down */' just above
the call.

The following is possible for EINVAL to be returned:

 1) hi - lo < page_size
 2) lenght = hi - lo; lo + lenght < lo => Unsigned integer overflow
 3) The memory range is locked.  See mlock(2)
 4) The variable `page_size' is zero for some reason (highly doubt)

You can check for all of the above with a debugger or `strace(1)'
easily.

Regards,
Olivier
-- 
Olivier Dion
oldiob.ca