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Re: [PATCH 2/6] x86/alternative: Walk all replacements in debug builds
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 26 Sep 2024 08:31:55 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 26 Sep 2024 06:32:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.09.2024 23:15, Andrew Cooper wrote:
> On 23/04/2024 3:44 pm, Jan Beulich wrote:
>> On 22.04.2024 20:14, Andrew Cooper wrote:
>>> + if ( res.rel_type == REL_TYPE_d8 )
>>> + {
>>> + int8_t *d8 = res.rel;
>>> + void *target = ip + res.len + *d8;
>>> +
>>> + if ( target < repl || target > end )
>>> + {
>>> + printk("Alternative for %ps [%*ph]\n",
>>> + ALT_ORIG_PTR(a), a->repl_len, repl);
>>> + panic("'JMP/Jcc disp8' at +%u leaves alternative
>>> block\n",
>>> + (unsigned int)(unsigned long)(ip - repl));
>>> + }
>>> + }
>> Why's Disp8 more important to check than Disp32? A bad CALL in a
>> replacement can't possibly be encoded with Disp8, and both JMP and Jcc
>> are also more likely to be encoded with Disp32 when their target isn't
>> in the same blob (but e.g. in a different section).
>
> Whatever the likelihood of them existing, Disp8's cannot possibly be
> correct if they cross the boundary of the replacement. Checking for
> them has the side effect of running decode_lite() over all replacements.
>
> Disp32's do exist in both external and internal forms (retpoline), and
> the point of this series is to make all external cases usable.
Okay, fine then.
> Therefore, there are no invalid cases.
There definitely are: Any pointing outside of the present replacement
block, into another replacement block. Which can in principle happen if
a label was used wrongly. Anything pointing outside the block really
needs to be covered by logic adjusting the displacement when the
alternative is being put in place.
Jan
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