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carpstats are the last virtualised variable in the file and end up at the

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end of the vnet_set.  The generated code uses an absolute relocation at
one byte beyond the end of the carpstats array.  This means the relocation
for the vnet does not happen for carpstats initialisation and as a result
the kernel panics on module load.

This problem has only been observed with carp and only on i386.
We considered various possible solutions including using linker scripts
to add padding to all kernel modules for pcpu and vnet sections.

While the symbols (by chance) stay in the order of appearance in the file
adding an unused non-file-local variable at the end of the file will extend
the size of set_vnet and hence make the absolute relocation for carpstats
work (think of this as a single-module set_vnet padding).

This is a (tmporary) hack.  It is the least intrusive one as we need a
timely solution for the upcoming release.  We will revisit the problem in
HEAD.  For a lot more information and the possible alternate solutions
please see the PR and the references therein.

PR:			230857
MFC after:		3 days
This commit is contained in:
Bjoern A. Zeeb 2018-11-01 17:26:18 +00:00
parent b4b90c1f4c
commit e2c532f156
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=340009
1 changed files with 15 additions and 0 deletions
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15
sys/netinet/ip_carp.c
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@ -2176,6 +2176,21 @@ static struct protosw in6_carp_protosw = {
};
#endif
#ifdef VIMAGE
#if defined(__i386__)
/*
* XXX This is a hack to work around an absolute relocation outside
* set_vnet by one (on the stop symbol) for carpstats. Add a dummy variable
* to the end of the file in the hope that the linker will just keep the
* order (as it seems to do at the moment). It is understood to be fragile.
* See PR 230857 for a longer discussion of the problem and the referenced
* review for possible alternate solutions. Each is a hack; we just need
* the least intrusive one for the next release.
*/
VNET_DEFINE(char, carp_zzz) = 0xde;
#endif
#endif
static void
carp_mod_cleanup(void)
{