[PATCH 1/5] rust: fix size_t in bindgen prototypes of C builtins

[Gary Guo <gary@garyguo.net>]

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.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/rust/Makefile b/rust/Makefile
index 4eae318f36ff7..863d87ad0ce68 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -265,7 +265,11 @@ 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__
 
 quiet_cmd_bindgen = BINDGEN $@
       cmd_bindgen = \
-- 
2.44.1