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patch-author-guide: Add section about patching syscalls 

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
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Josh Poimboeuf 2022-04-22 08:36:11 -07:00
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commit fe5df04d30
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doc/patch-author-guide.md
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@ -874,3 +874,14 @@ To track down specifically what caused a symbol CRC change, tools like
detailed symbol definition report. For a kpatch-build, kabi-dw can be modified
to operate on .o object files (not just .ko and vmlinux files) and the
`$CACHEDIR/tmp/{orig, patched}` directories compared.
System calls
------------
Attempting to patch a syscall typically results in an error, due to a missing
fentry hook in the inner `__do_sys##name()` function. The fentry hook is
missing because of the 'inline' annotation, which invokes 'notrace'.
This problem can be worked around by adding `#include "kpatch-syscall.h"` and
replacing the use of the `SYSCALL_DEFINE1` (or similar) macro with the
`KPATCH_` prefixed version.