[PATCH] lib/base64: Silence clang-24 -Wconstant-conversion with diag pragmas

[Nathan Chancellor <nathan@kernel.org>]

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.

Cc: stable@vger.kernel.org
Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")
Closes: https://github.com/ClangBuiltLinux/linux/issues/2181
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]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 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

---
base-commit: 0c2669a9f4a1d607e7591ae50ccf3c432a0aff08
change-id: 20261008-base64-silence-clang-24-constant-conversion-b682c6bbf563

Best regards,
--  
Cheers,
Nathan