From 3af83c11cf5990fda0bfe3959aa6391ff903ffdd Mon Sep 17 00:00:00 2001 From: Nathan Chancellor Date: Thu, 8 Oct 2026 13:08:49 +0200 Subject: [PATCH] lib/base64: silence clang-24 -Wconstant-conversion with diag pragmas After a recent change in clang to warn on signed char conversions within array initializers [1], there are several instances of this warning from base64_rev_maps in lib/base64.c, which can break the build with W=e or CONFIG_WERROR=y: lib/base64.c:58:18: error: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 131 to -125 [-Werror,-Wconstant-conversion] 58 | [BASE64_IMAP] = BASE64_REV_INIT('+', ',') | ^~~~~~~~~~~~~~~~~~~~~~~~~ lib/base64.c:52:2: note: expanded from macro 'BASE64_REV_INIT' 48 | #define BASE64_REV_INIT(ch_62, ch_63) { \ | ~ 49 | [0 ... 0x1f] = -1, \ 50 | INIT_32(0x20, ch_62, ch_63), \ 51 | INIT_32(0x40, ch_62, ch_63), \ 52 | INIT_32(0x60, ch_62, ch_63), \ | ^~~~~~~~~~~~~~~~~~~~~~~~~~~ ... lib/base64.c:34:42: note: expanded from macro 'INIT_1' 34 | : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \ | ~~~~~~~~~~^~~~ lib/base64.c:58:18: error: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 130 to -126 [-Werror,-Wconstant-conversion] 58 | [BASE64_IMAP] = BASE64_REV_INIT('+', ',') | ^~~~~~~~~~~~~~~~~~~~~~~~~ lib/base64.c:52:2: note: expanded from macro 'BASE64_REV_INIT' 48 | #define BASE64_REV_INIT(ch_62, ch_63) { \ | ~ 49 | [0 ... 0x1f] = -1, \ 50 | INIT_32(0x20, ch_62, ch_63), \ 51 | INIT_32(0x40, ch_62, ch_63), \ 52 | INIT_32(0x60, ch_62, ch_63), \ | ^~~~~~~~~~~~~~~~~~~~~~~~~~~ ... lib/base64.c:34:42: note: expanded from macro 'INIT_1' 34 | : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \ | ~~~~~~~~~~^~~~ ... These are false positives, as the branch where the wraparound could happen is unreachable with the values that clang reports. This is not considered a bug by some clang folks [2][3], so silence the warnings using the __diag macros the kernel has to workaround compiler warnings when necessary. Link: https://lore.kernel.org/20261008-base64-silence-clang-24-constant-conversion-v1-1-0858b60b23c8@kernel.org Link: https://github.com/llvm/llvm-project/commit/a5ef934a8d295dc03be3960f2b3744ec2e53238e [1] Link: https://github.com/llvm/llvm-project/issues/223923#issuecomment-6056856245 [2] Link: https://github.com/llvm/llvm-project/pull/226775#pullrequestreview-5454407389 [3] Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables") Signed-off-by: Nathan Chancellor Signed-off-by: Andrew Morton Closes: https://github.com/ClangBuiltLinux/linux/issues/2181 Acked-by: Kuan-Wei Chiu Cc: Nick Desaulniers Cc: Bill Wendling Cc: Justin Stitt Cc: Guan-Chun Wu <409411716@gms.tku.edu.tw> Cc: --- lib/base64.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/lib/base64.c b/lib/base64.c index 325c7332b049..e46be3a55585 100644 --- a/lib/base64.c +++ b/lib/base64.c @@ -52,11 +52,14 @@ static const char base64_tables[][65] = { INIT_32(0x60, ch_62, ch_63), \ [0x80 ... 0xff] = -1 } +__diag_push(); +__diag_ignore(clang, all, "-Wconstant-conversion", "https://github.com/llvm/llvm-project/issues/223923"); static const s8 base64_rev_maps[][256] = { [BASE64_STD] = BASE64_REV_INIT('+', '/'), [BASE64_URLSAFE] = BASE64_REV_INIT('-', '_'), [BASE64_IMAP] = BASE64_REV_INIT('+', ',') }; +__diag_pop(); #undef BASE64_REV_INIT #undef INIT_32