Allow C99 flexible array members in unions and alone in structs like 0-length arrays #84565
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c99
clang:frontend
Language frontend issues, e.g. anything involving "Sema"
extension:gnu
extension:microsoft
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EugeneZelenko
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clang:frontend
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@llvm/issue-subscribers-clang-frontend Author: Kees Cook (kees)
GNU and MSVC have extensions where flexible array members (or their equivalent) can be in unions or alone in structs. This is already fully supported in Clang through the 0-sized array ("fake flexible array") extension or when C99 flexible array members have been syntactically obfuscated.
Please explicitly allow these extensions directly for C99 flexible arrays, since they are common code patterns in active use by the Linux kernel (and other projects). Such projects have been using either 0-sized arrays (which is considered deprecated in favor of C99 flexible array members) or via obfuscated syntax, both of which complicate their code bases. For example, these do not error by default: But this does: Matching request has been made to GCC as well: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548 @nickdesaulniers |
kees
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GNU and MSVC have extensions where flexible array members (or their
equivalent) can be in unions or alone in structs. This is already fully
supported in Clang through the 0-sized array ("fake flexible array")
extension or when C99 flexible array members have been syntactically
obfuscated.
Clang needs to explicitly allow these extensions directly for C99
flexible arrays, since they are common code patterns in active use by the
Linux kernel (and other projects). Such projects have been using either
0-sized arrays (which is considered deprecated in favor of C99 flexible
array members) or via obfuscated syntax, both of which complicate their
code bases.
For example, these do not error by default:
union one {
int a;
int b[0];
};
union two {
int a;
struct {
struct { } __empty;
int b[];
};
};
But this does:
union three {
int a;
int b[];
};
Remove the default error diagnostics for this but continue to provide
warnings under Microsoft or GNU extensions checks. This will allow for
a seamless transition for code bases away from 0-sized arrays without
losing existing code patterns. Add explicit checking for the warnings
under various constructions.
Fixes llvm#84565
|
PR #84428 has been proposed to fix this. |
kees
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Mar 27, 2024
…84428) GNU and MSVC have extensions where flexible array members (or their equivalent) can be in unions or alone in structs. This is already fully supported in Clang through the 0-sized array ("fake flexible array") extension or when C99 flexible array members have been syntactically obfuscated. Clang needs to explicitly allow these extensions directly for C99 flexible arrays, since they are common code patterns in active use by the Linux kernel (and other projects). Such projects have been using either 0-sized arrays (which is considered deprecated in favor of C99 flexible array members) or via obfuscated syntax, both of which complicate their code bases. For example, these do not error by default: ``` union one { int a; int b[0]; }; union two { int a; struct { struct { } __empty; int b[]; }; }; ``` But this does: ``` union three { int a; int b[]; }; ``` Remove the default error diagnostics for this but continue to provide warnings under Microsoft or GNU extensions checks. This will allow for a seamless transition for code bases away from 0-sized arrays without losing existing code patterns. Add explicit checking for the warnings under various constructions. Additionally fixes a CodeGen bug with flexible array members in unions in C++, which was found when adding a testcase for: ``` union { char x[]; } z = {0}; ``` which only had Sema tests originally. Fixes #84565
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GNU and MSVC have extensions where flexible array members (or their equivalent) can be in unions or alone in structs. This is already fully supported in Clang through the 0-sized array ("fake flexible array") extension or when C99 flexible array members have been syntactically obfuscated.
Please explicitly allow these extensions directly for C99 flexible arrays, since they are common code patterns in active use by the Linux kernel (and other projects). Such projects have been using either 0-sized arrays (which is considered deprecated in favor of C99 flexible array members) or via obfuscated syntax, both of which complicate their code bases.
For example, these do not error by default:
But this does:
Matching request has been made to GCC as well: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
@nickdesaulniers
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