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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
parent b21b35f7d4
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 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 to operate on .o object files (not just .ko and vmlinux files) and the
`$CACHEDIR/tmp/{orig, patched}` directories compared. `$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.