From 999bea31a252dae1b0709166cf3c9540bda9dbb0 Mon Sep 17 00:00:00 2001 From: Xavier Leroy Date: Fri, 17 Aug 2018 10:35:59 +0200 Subject: Issue with packed structs and sizeof, alignof, offsetof in cparser/ CompCert has two implementations of sizeof, alignof and offsetof (byte offset of a struct field): - the reference implementation, in Coq, from cfrontend/Ctypes.v - the implementation used during elaboration, in OCaml, from cparser/Cutil.ml The reference Coq implementation is used as much as possible, but sometimes during elaboration the size of a type must be computed (e.g. to compute array sizes), or the offset of a field (e.g. to evaluate __builtin_offsetof), in which case the OCaml implementation is used. This causes issues with packed structs. Currently, the cparser/Cutil.ml functions ignore the "packed" attribute on structs. Their results disagree with the "true" sizes, alignments and offsets computed by the cfrontend/Ctypes.v functions after source-to-source transformation of packed structs as done in cparser/PackedStruct.ml. For example: ``` struct __packed__(1) s { char c; short s; int i; }; assert (__builtin_offsetof(struct s, i) == 3); assert (sizeof(struct s) = sizeof(char[sizeof(struct s)])); ``` The two assertions fail. In the first assertion, __builtin_offsetof is elaborated to 4, because the packed attribute is ignored during elaboration. In the second assertion, the type `char[sizeof(struct s)]` is elaborated to `char[8]`, again because the packed attribute is ignored during elaboration, while the other `sizeof(struct s)` is computed as 7 after the source-to-source transformation of packed structs. This commit changes the cparser/Cutil.ml functions so that they take the packed attribute into account when computing sizeof, alignof, offsetof, and struct_layout. Related changes: * cparser/Cutil: add `packing_parameters` function to extract packing info from attributes * cparser/Cutil: refactor and share more code between sizeof_struct, offsetof, and struct_layout * cparser/Elab: check the alignment parameters given in packed attributes. (The check was previously done in cparser/PackedStruct.ml but now it would come too late.) * cparser/Elab: refactor the checking of alignment parameters between _Alignas, attribute((aligned)), __packed__, and attribute((packed)). * cparser/PackedStructs: simplify the code, some functionality was moved to cparser/Cutil, other to cparser/Elab * cfrontend/C2C: raise an "unsupported" error if a packed struct is defined and -fpacked-structs is not given. Before, the packed attribute would be silently ignored, but now doing so would cause inconsistencies between cfrontend/ and cparser/. * test/regression/packedstruct1.c: add tests to compare the sizes and the offsets produced by the elaborator with those obtained after elaboration. --- cparser/Cutil.mli | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) (limited to 'cparser/Cutil.mli') diff --git a/cparser/Cutil.mli b/cparser/Cutil.mli index fb875b0d..5ae5dcd7 100644 --- a/cparser/Cutil.mli +++ b/cparser/Cutil.mli @@ -36,6 +36,11 @@ val incl_attributes : attributes -> attributes -> bool val alignas_attribute : attributes -> int (* Extract the value of the [_Alignas] attributes, if any. Return 0 if none, a (positive) power of two alignment if some. *) +val packing_parameters : attributes -> int * int * bool + (* Extract the value of the [__packed__] attributes, if any. + Return a triple + (maximum field alignment, minimum struct alignment, byte swapping). + Alignments of 0 mean default alignment. *) val find_custom_attributes : string list -> attributes -> attr_arg list list (* Extract arguments of custom [Attr] attributes whose names appear in the given list of names. *) @@ -127,7 +132,7 @@ val composite_info_decl: val composite_info_def: Env.t -> struct_or_union -> attributes -> field list -> Env.composite_info val struct_layout: - Env.t -> field list -> (string * int) list + Env.t -> attributes -> field list -> (string * int) list val offsetof: Env.t -> typ -> field -> int (* Compute the offset of a struct member *) -- cgit