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Jose E. Marchesi
ff2071a7b7
The generated bpf_helper_defs.h file currently contains definitions like this for the kernel helpers, which are static objects: static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1; These work well in both clang and GCC because both compilers do constant propagation with -O1 and higher optimization, resulting in `call 1' BPF instructions being generated, which are calls to kernel helpers. However, there is a discrepancy on how the -Wunused-variable warning (activated by -Wall) is handled in these compilers: - clang will not emit -Wunused-variable warnings for static variables defined in C header files, be them constant or not constant. - GCC will not emit -Wunused-variable warnings for _constant_ static variables defined in header files, but it will emit warnings for non-constant static variables defined in header files. There is no reason for these bpf_helpers_def.h pointers to not be declared constant, and it is actually desirable to do so, since their values are not to be changed. So this patch modifies bpf_doc.py to generate prototypes like: static void *(* const bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1; This allows GCC to not error while compiling BPF programs with `-Wall -Werror', while still being able to detect and error on legitimate unused variables in the program themselves. This change doesn't impact the desired constant propagation in neither Clang nor GCC with -O1 and higher. On the contrary, being declared as constant may increase the odds they get constant folded when used/referred to in certain circumstances. Tested in bpf-next master. No regressions. Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Acked-by: Yonghong Song <yonghong.song@linux.dev> Link: https://lore.kernel.org/bpf/20240127185031.29854-1-jose.marchesi@oracle.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.