From eab63b5a8cf2214ddfee566a87deb3013ffcc362 Mon Sep 17 00:00:00 2001 From: Michael Jones Date: Wed, 25 Sep 2024 15:30:47 -0700 Subject: [PATCH] [libc] Fix %m on CPUs with float128 but no int128 (#110053) This bug is caused by the BigInt implementation failing to initialize from errno. Explanation below, but the fix is a static cast to int. The bug only shows up on risc-v 32 because of a chain of type-oddities: 1) Errno is provided by a struct with an implicit cast to int. 2) The printf parser uses an int128 to store the value of a conversion on systems with long double greater than double. 3) On systems without native int128 support we use our own BigInt instead. These combine such that if both long double and int128 exist (e.g. on x86) there's no issue, errno is implicitly cast to int, which is extended to int128. If long double is double (e.g. on arm32) then int64 is used in the printf parser, the implicit cast works, and there's no issue. The only way this would come up is if the target has a proper long double type, but not int128, which is the case for at least the current risc-v 32 bot. --- libc/src/stdio/printf_core/parser.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libc/src/stdio/printf_core/parser.h b/libc/src/stdio/printf_core/parser.h index e2cb734b5be7..acbbaa25b1c9 100644 --- a/libc/src/stdio/printf_core/parser.h +++ b/libc/src/stdio/printf_core/parser.h @@ -265,7 +265,7 @@ public: case ('m'): // %m is an odd conversion in that it doesn't consume an argument, it // just takes the current value of errno as its argument. - section.conv_val_raw = libc_errno; + section.conv_val_raw = static_cast(libc_errno); break; #endif // LIBC_COPT_PRINTF_DISABLE_STRERROR #ifndef LIBC_COPT_PRINTF_DISABLE_WRITE_INT