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lib/base64: silence clang-24 -Wconstant-conversion with diag pragmas

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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: a5ef934a8d [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: c4eb7ad32e ("lib/base64: optimize base64_decode() with reverse lookup tables")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Closes: https://github.com/ClangBuiltLinux/linux/issues/2181
Acked-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Guan-Chun Wu <409411716@gms.tku.edu.tw>
Cc: <stable@vger.kernel.org>
This commit is contained in:
Nathan Chancellor 2026-10-08 13:08:49 +02:00 committed by Andrew Morton
parent 6628e7d4c6
commit 3af83c11cf
1 changed files with 3 additions and 0 deletions
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3
lib/base64.c
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@ -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