[Crash-utility] [PATCH 1/1] Calculate offset to field 'init_uts_ns.name'

[HAGIO KAZUHITO(萩尾 一仁)]

-----Original Message-----
> On Thu, 1 Oct 2020 00:38:23 +0000
> HAGIO KAZUHITO(萩尾　一仁) <k-hagio-ab at nec.com> wrote:
> 
> > Hi Alex,
> >
> > sorry for the delayed response.
> >
> > I misunderstood at first glance and have waited for the kernel patch
> > adding the vmcoreinfo entry you posted, but I just found no need to
> > wait for it with respect to crash.
> >
> > -----Original Message-----
> > > The offset has changed in linux-next (v5.9.0) from 4 to 0 because
> > > there is no more 'kref' member variable at the beginning of 'init_uts_ns'.
> > > The change was introduced with commit 9a56493f6942c0e2df1579986128721da96e00d8.
> > > To handle both cases correctly, calculate the offset at run time instead.
> > >
> > > Signed-off-by: Alexander Egorenkov <egorenar at linux.ibm.com>
> > > ---
> > >  kernel.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel.c b/kernel.c
> > > index 21bd1eb..13942b9 100644
> > > --- a/kernel.c
> > > +++ b/kernel.c
> > > @@ -237,7 +237,7 @@ kernel_init()
> > >                  	sizeof(struct new_utsname), "system_utsname",
> > >  			RETURN_ON_ERROR);
> > >  	else if (symbol_exists("init_uts_ns"))
> > > -		readmem(symbol_value("init_uts_ns") + sizeof(int),
> > > +		readmem(symbol_value("init_uts_ns") + ANON_MEMBER_OFFSET("uts_namespace","name"),
> >
> > When we get an offset from vmlinux, usually we introduce a member to
> > struct offset_table (at the tail of it) and populate it with MEMBER_OFFSET_INIT()
> > macro so that we can reuse it and dump it with dump_offset_table() function.
> > Can you please update so?
> >
> > For reference:
> > https://www.redhat.com/archives/crash-utility/2020-August/msg00052.html
> >
> > and fyi, crash doesn't use the vmcoreinfo entry OFFSET(uts_namespace.name)
> > with respect to this case.
> 
> FYI here's how libkdumpfile deals with the issue:
> 
> https://github.com/ptesarik/libkdumpfile/blob/tip/src/kdumpfile/open.c#L129
> 
> Essentially, we can only handle Linux kernel dumps anyway, so sysname
> is always set to "Linux". You can search for this "magic" string, and
> then you have the address of init_uts_ns.name.sysname. Calculating t
> address of init_uts_ns.name is then trivial.

Hi Petr,

thanks for the info, memorized.
I'd like to use the orthodox method first as usual, but makedumpfile
(rather than crash) might need such a tool side workaround in future.

Thanks,
Kazu