Re: [PATCH] powerpc: Use sysconf (_SC_SIGSTKSZ) to set SIGSTKSZ and MINSIGSTKSZ.

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, May 17, 2023 at 3:18 PM Rajalakshmi Srinivasaraghavan
<rajis@linux.vnet.ibm.com> wrote:
>
>
> On 5/11/23 11:50 AM, Manjunath S Matti wrote:
>
>
> On 09/05/23 11:03 pm, Tulio Magno Quites Machado Filho wrote:
>
> Manjunath S Matti <mmatti@linux.vnet.ibm.com> writes:
>
> Am I missing some thing, please help me understand the file
>
> No. You're right.
> The issue I hypothesized can't happen.
>
> Thanks for confirming.
>
> The patch looks good.
>
> I will definitely add a testcases just to check what value are we getting
>
> from the kernel.
>
> I'm sorry, I'm not sure this test is needed anymore.
> While it doesn't hurt to have it, it would add little value.
>
> Thanks again!
>
> I have one last thing that I fail to understand, in x86
>
> file: sysdeps/unix/sysv/linux/bits/sigstksz.h
>
>  26 /* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
>  27 # undef SIGSTKSZ
>  28 # define SIGSTKSZ sysconf (_SC_SIGSTKSZ)
>  29
>  30 /* Minimum stack size for a signal handler: SIGSTKSZ.  */
>  31 # undef MINSIGSTKSZ
>  32 # define MINSIGSTKSZ SIGSTKSZ
>
> line number 32, both MINSIGSTKSZ and SIGSTKSZ are intentionally set to the same value.
>
> Do you know why, I just wanted to know the reason behind this.
>
>
> This was added by commit 6c57d320484988e87e446e2e60ce42816bf51d53.
>
> @H.J. Lu Do you have any comments on this question?
>

There are

@item _SC_MINSIGSTKSZ
@standards{GNU, unistd.h}
Inquire about the minimum number of bytes of free stack space required
in order to guarantee successful, non-nested handling of a single signal
whose handler is an empty function.

@item _SC_SIGSTKSZ
@standards{GNU, unistd.h}
Inquire about the suggested minimum number of bytes of stack space
required for a signal stack.

This is not guaranteed to be enough for any specific purpose other than
the invocation of a single, non-nested, empty handler, but nonetheless
should be enough for basic scenarios involving simple signal handlers
and very low levels of signal nesting (say, 2 or 3 levels at the very
most).

This value is provided for developer convenience and to ease migration
from the legacy @code{SIGSTKSZ} constant.  Programs requiring stronger
guarantees should avoid using it if at all possible.

@vtable @code
@item SIGSTKSZ
This is the canonical size for a signal stack.  It is judged to be
sufficient for normal uses.

@item MINSIGSTKSZ
This is the amount of signal stack space the operating system needs just
to implement signal delivery.  The size of a signal stack @strong{must}
be greater than this.

For most cases, just using @code{SIGSTKSZ} for @code{ss_size} is
sufficient.  But if you know how much stack space your program's signal
handlers will need, you may want to use a different size.  In this case,
you should allocate @code{MINSIGSTKSZ} additional bytes for the signal
stack and increase @code{ss_size} accordingly.

Since sysconf (_SC_SIGSTKSZ) is the suggested minimum signal stack size,
it is used for both SIGSTKSZ and MINSIGSTKSZ.


-- 
H.J.