support .bss.* bundling

Allow bundling of .bss.* sections that are the result of -fdata-sections
so that rela sections referencing data in bss sections by section symbol
can be replaced with the object symbol so it can be linked to the existing
data object in the kernel.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
use d_size instead of sh_size

The section header size is calculated at output time by libelf
and we use it as a read-only value from read files.

With the next patch we are changing the size of the .rela__bug_table
section.  Lets use d_size instead since it is the value that tells
libelf how to calculate sh_size at output time.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
add bug table support

This commit adds a new function to properly handle the bug table.
It works by going through .rela__bug_table, after the changed
function symbols have already been marked, and rewrites the section
including only the relocations pertaining to bug entries for
changed functions.

The __bug_table section itself is not modified resulting in
"blank" bug entries: ones whose IP and filename pointers will
not be relocated and, therefore, will be zero.  While a waste
of space, it simplifies the code not to remove these blank
entries. They do no harm.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
add info to expected rela sym error

While debugging the code for the bug table logic, I found it useful to
know which rela section and entry the error occurred on.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
kpatch: look for core module in git dir

Allow "kpatch load" to find the core module when kpatch is run directly
from the git tree.  This gives the user the option to use the kpatch
utility directly without having to do a "make install".
readme: recommend use of the kpatch utility

The kpatch utility is now user friendly enough that it can be used
instead of direct insmods.  We should encourage people to use it, since
we will soon be adding user space functionality above and beyond
insmod/rmmod when loading and unloading.
fixup review comments

Signed-off-by: Seth Jennings <sjenning@redhat.com>
Merge pull request #134 from jpoimboe/find-core-module-git-tree

kpatch: look for core module in git dir
Merge pull request #133 from spartacus06/bugtable

Add support for bug table and data in .bss sections
kmod/core: Handle registering error and unroll it

Handle registering error to unroll the ftrace filter.
This also introduces get_kpatch_func() and
kpatch_remove_funcs_from_filter() for holding up
redundant loops.

Changes from v2:
 - Rebased on the latest kpatch.

Changes from v1:
 - Rename get_kpatch_func to kpatch_get_func.
 - Fix function definition style issue.
 - Do not jump to a label in "if" block.
 - Rollback the ftrace user counter if we hit an error.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
kmod/core: Support live patching on NMI handlers

Support live patching on NMI handlers. This adds checks for
possible inconsistency of live patching on NMI handlers.
The inconsistency problem means that any concurrent execution
of old function and new function, which can lead unexpected results.

Current kpatch checks possible inconsistency problem with
stop_machine, which can cover only threads and normal interrupts.
However, beacuse NMI can not stop with it, stop_machine is not
enough for live patching on NMI handlers or sub-functions which are
invoked in the NMI context.

To check for possible inconsistency of live patching on those
functions, add an atomic flag to count patching target functions
invoked in NMI context while updating kpatch hash table. If the
flag is set by the target functions in NMI, we can not ensure
there is no concurrent execution on it.

This fixes the issue #65.

Changes from v5:
 - Fix to add a NULL check in kpatch_get_committed_func().

Changes from v4:
 - Change kpatch_operation to atomic_t.
 - Use smp_rmb/wmb barriers between kpatch_operation and kpatch_status.
 - Check in_nmi() first and if true, access kpatch_operation.

Changes from v3:
 - Fix kpatch_apply/remove_patch to return 0 if succeeded.

Changes from v2:
 - Clean up kpatch_get_committed_func as same style of kpatch_get_func.
 - Rename opr to op in kpatch_ftrace_handler.
 - Consolidate in_nmi() and kpatch_operation check into one condition.
 - Fix UNPATCH/PATCH mistype in kpatch_register.

Changes from v1:
 - Rename inconsistent_flag to kpatch_status.
 - Introduce new enums and helper functions for kpatch_status.
 - Use hash_del_rcu instead of hlist_del_rcu.
 - Rename get_committed_func to kpatch_get_committed_func.
 - Use ACCESS_ONCE for kpatch_operation to prevent compiler optimization.
 - Fix to remove (!func || func->updating) condition from NMI check.
 - Add more precise comments.
 - Fix setting order of kpatch_status and kpatch_operation.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>