Re: [Xen-devel] [PATCH 2/2] xen/livepatch: Don't crash on encountering STN_UNDEF relocations

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/06/17 21:51, Andrew Cooper wrote:
> A symndx of STN_UNDEF is special, and means a symbol value of 0.
>
> There is no real symbol data for it, so avoid tripping over a NULL pointer
> with "elf->sym[symndx].sym->st_value".
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
>
> Functionally tested on x86, but both arm variants look to suffer from the same
> issue.  Compile tested on all architectures.
>
> TODO: Figure out how my livepatch has a STN_UNDEF relocation...

On second thoughts, maybe STN_UNDEF symbols should be a hard failure.

(XEN) livepatch: live: Applying 1 functions
(XEN) ----[ Xen-4.10-unstable  x86_64  debug=y   Not tainted ]----
(XEN) CPU:    3
(XEN) RIP:    e008:[<0000000000000000>] 0000000000000000

In this case, the hook function hasn't been wired up correctly, which
causes apply_payload() to fall over a NULL data->load_funcs[i]();

As for why the STN_UNDEF symbol, it comes from an assembly hook function
missing a .type attribute.  Still, Xen shouldn't crash when it
encounters one.

~Andrew

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