[Crash-utility] [PATCH] cmdline: Add a new "--machdep stacksize=<value>".

[Sean Fu]

On Mon, Oct 01, 2018 at 09:37:10AM -0400, Dave Anderson wrote:
> 
> 
> ----- Original Message -----
> > Implemented support for 16k stack size that was introduced by commit
> > 6538b8ea886e472f4431db8ca1d60478f838d14b titled "x86_64: expand kernel
> > stack to 16K".
> > Without the patch, kernels has 16k stack, leading to errors in commands
> > such as "bt" and any command regarding 8K stack.
> > Add a new "--machdep stacksize=<value>" option that can be used to
> > override the default machdep->stacksize value which is 8k.
> 
> The x86_64 default value of 8K is basically a leftover value that each of 
> the architectures originally used for setting machdep->stacksize.  But for 
> quite some time now, those values should get overridden later on here
> in task_init():
> 
>         STRUCT_SIZE_INIT(task_union, "task_union");
>         STRUCT_SIZE_INIT(thread_union, "thread_union");
> 
>         if (VALID_SIZE(task_union) && (SIZE(task_union) != STACKSIZE())) {
>                 error(WARNING, "\nnon-standard stack size: %ld\n",
>                         len = SIZE(task_union));
>                 machdep->stacksize = len;
>         } else if (VALID_SIZE(thread_union) &&
>                 ((len = SIZE(thread_union)) != STACKSIZE())) {
>                 machdep->stacksize = len;
>         } else if (!VALID_SIZE(thread_union) && !VALID_SIZE(task_union)) {
>                 if (kernel_symbol_exists("__start_init_task") &&
>                     kernel_symbol_exists("__end_init_task")) {
>                         len = symbol_value("__end_init_task");
>                         len -= symbol_value("__start_init_task");
>                         ASSIGN_SIZE(thread_union) = len;
>                         machdep->stacksize = len;
>                 }
>         }
> 
I compiled latest kernel and latest crash and run a qemu guest machine with the latest compliled kernel
image.
In this case, STRUCT_SIZE_INIT initialized size_table.task_union and
size_table.thread_union with -1. So machdep->stacksize did NOT get
overridden.
> As of Linux 4.18 at least, x86_64 still uses the thread_union declaration.
> For example:
> 
>   crash> thread_union
>   union thread_union {
>       struct task_struct task;
>       unsigned long stack[2048];
>   }
>   SIZE: 16384
>   crash>
> 
> On what kernel version are you seeing the obsolete 8k stacksize being used?
> What does the command above show on your system?
kernel version is upstream Linux 4.18 (commit#94710cac0ef4ee177a63b5227664b38c95bbf703)
(git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git).

"bt" command in crash shows "bt: invalid RSP: ffffc9000069bc08
bt->stackbase/stacktop: ffffc90000698000/ffffc9000069a000 cpu: 0".

BestRegards
Sean
> 
> Thanks,
>   Dave
> 
> 
> 
>  
> > 
> > Signed-off-by: Sean Fu <fxinrong at gmail.com>
> > ---
> >  x86_64.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/x86_64.c b/x86_64.c
> > index 7d01140..1798f05 100644
> > --- a/x86_64.c
> > +++ b/x86_64.c
> > @@ -5716,6 +5716,15 @@ parse_cmdline_args(void)
> >  						continue;
> >  					}
> >  				}
> > +			} else if (STRNEQ(arglist[i], "stacksize=")) {
> > +	                        p = arglist[i] + strlen("stacksize=");
> > +				if (strlen(p)) {
> > +					value = stol(p, RETURN_ON_ERROR|QUIET, &errflag);
> > +					if (!errflag) {
> > +						machdep->stacksize = value;
> > +						continue;
> > +					}
> > +				}
> >  			}
> >  	
> >  			error(WARNING, "ignoring --machdep option: %s\n", arglist[i]);
> > --
> > 2.6.2
> > 
> >