selftests/bpf: Test syscall command parsing

Add building of the bpf(2) syscall commands documentation as part of the
docs building step in the build. This allows us to pick up on potential
parse errors from the docs generator script as part of selftests.

The generated manual pages here are not intended for distribution, they
are just a fragment that can be integrated into the other static text of
bpf(2) to form the full manual page.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-14-joe@cilium.io

tools/testing/selftests/bpf/.gitignore

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 bpf-helpers*
+bpf-syscall*
 test_verifier
 test_maps
 test_lru_map

tools/testing/selftests/bpf/Makefile.docs

@@ -15,18 +15,27 @@ endif
 
 prefix ?= /usr/local
 mandir ?= $(prefix)/man
+man2dir = $(mandir)/man2
 man7dir = $(mandir)/man7
 
+SYSCALL_RST = bpf-syscall.rst
+MAN2_RST = $(SYSCALL_RST)
+
 HELPERS_RST = bpf-helpers.rst
 MAN7_RST = $(HELPERS_RST)
 
+_DOC_MAN2 = $(patsubst %.rst,%.2,$(MAN2_RST))
+DOC_MAN2 = $(addprefix $(OUTPUT),$(_DOC_MAN2))
+
 _DOC_MAN7 = $(patsubst %.rst,%.7,$(MAN7_RST))
 DOC_MAN7 = $(addprefix $(OUTPUT),$(_DOC_MAN7))
 
-DOCTARGETS := helpers
+DOCTARGETS := helpers syscall
 
 docs: $(DOCTARGETS)
+syscall: man2
 helpers: man7
+man2: $(DOC_MAN2)
 man7: $(DOC_MAN7)
 
 RST2MAN_DEP := $(shell command -v rst2man 2>/dev/null)
@@ -64,9 +73,10 @@ endef
 
 # Create the make targets to generate manual pages by name and section
 $(eval $(call DOCS_RULES,helpers,7))
+$(eval $(call DOCS_RULES,syscall,2))
 
 docs-clean: $(foreach doctarget,$(DOCTARGETS), docs-clean-$(doctarget))
 docs-install: $(foreach doctarget,$(DOCTARGETS), docs-install-$(doctarget))
 docs-uninstall: $(foreach doctarget,$(DOCTARGETS), docs-uninstall-$(doctarget))
 
-.PHONY: docs docs-clean docs-install docs-uninstall man7
+.PHONY: docs docs-clean docs-install docs-uninstall man2 man7