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

[Sean Fu]

On Tue, Oct 09, 2018 at 09:39:10AM -0400, Dave Anderson wrote:
> 
> 
> ----- Original Message -----
> > 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
> 
> 
> 
> Ok, the most recent 4.18 kernel I have on hand is this one:
> 
>   crash> sys | grep RELEASE
>        RELEASE: 4.18.0-20.el8.x86_64
>   crash>
> 
> and its debuginfo data contains the "thread_union" information:
> 
>   crash> thread_union
>   union thread_union {
>       struct task_struct task;
>       unsigned long stack[2048];
>   }
>   SIZE: 16384
>   crash>
> 
> but if it did not, then code should then calculate the stack
> size from the difference between the "__start_init_task" and 
> "__end_init_task" symbols:
> 
>   crash> sym __start_init_task
>   ffffffffa7800000 (D) __start_init_task
>   crash> sym __end_init_task
>   ffffffffa7804000 (D) __end_init_task
>   crash> 
>   
> Does your kernel not show/contain those 2 symbols?
>
Sure, my test kernel contains these 2 symbols.

crash> sys | grep RELEASE
     RELEASE: 4.18.0-1-default+
crash> thread_union
crash: command not found: thread_union
crash> struct thread_union
struct: invalid data structure reference: thread_union
crash> sym __start_init_task
ffffffff82000000 (D) __start_init_task
crash> sym __end_init_task
ffffffff82004000 (D) __end_init_task

Agree with you, Automatic calculation stack size from  the difference between__start_init_task and __end_init_task should be better.
Calculating and assignning stack size should be add into "x86_64_init", Do you think so?

Thanks
Sean
> Dave
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> > > 
> > > 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
> > > > 
> > > > 
> >