6.12-stable review patch. If anyone has any objections, please let me know.
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From: Gary Guo gary@garyguo.net
commit 75c1fd41a671a0843b89d1526411a837a7163fa2 upstream.
Without `-fno-builtin`, for functions like memcpy/memmove (and many others), bindgen seems to be using the clang-provided prototype. This prototype is ABI-wise compatible, but the issue is that it does not have the same information as the source code w.r.t. typedefs.
For example, bindgen generates the following:
extern "C" { pub fn strlen(s: *const core::ffi::c_char) -> core::ffi::c_ulong; }
note that the return type is `c_ulong` (i.e. unsigned long), despite the size_t-is-usize behavior (this is default, and we have not opted out from it using --no-size_t-is-usize).
Similarly, memchr's size argument should be of type `__kernel_size_t`, but bindgen generates `c_ulong` directly.
We want to ensure any `size_t` is translated to Rust `usize` so that we can avoid having them be different type on 32-bit and 64-bit architectures, and hence would require a lot of excessive type casts when calling FFI functions.
I found that this bindgen behavior (which probably is caused by libclang) can be disabled by `-fno-builtin`. Using the flag for compiled code can result in less optimisation because compiler cannot assume about their properties anymore, but this should not affect bindgen.
[ Trevor asked: "I wonder how reliable this behavior is. Maybe bindgen could do a better job controlling this, is there an open issue?".
Gary replied: ..."apparently this is indeed the suggested approach in https://github.com/rust-lang/rust-bindgen/issues/1770". - Miguel ]
Signed-off-by: Gary Guo gary@garyguo.net Link: https://lore.kernel.org/r/20240913213041.395655-2-gary@garyguo.net [ Formatted comment. - Miguel ] Signed-off-by: Miguel Ojeda ojeda@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- rust/Makefile | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
--- a/rust/Makefile +++ b/rust/Makefile @@ -264,7 +264,11 @@ else bindgen_c_flags_lto = $(bindgen_c_flags) endif
-bindgen_c_flags_final = $(bindgen_c_flags_lto) -D__BINDGEN__ +# `-fno-builtin` is passed to avoid `bindgen` from using `clang` builtin +# prototypes for functions like `memcpy` -- if this flag is not passed, +# `bindgen`-generated prototypes use `c_ulong` or `c_uint` depending on +# architecture instead of generating `usize`. +bindgen_c_flags_final = $(bindgen_c_flags_lto) -fno-builtin -D__BINDGEN__
# Each `bindgen` release may upgrade the list of Rust target versions. By # default, the highest stable release in their list is used. Thus we need to set