(The following sample is taken from my LKML post at
https://lkml.org/lkml/2023/11/15/213)
$ cat t-build-bug.c
struct vring_tx_mac {
unsigned int d[3];
unsigned int ucode_cmd;
} __attribute__((packed));
struct vring_rx_mac {
unsigned int d0;
unsigned int d1;
unsigned short w4;
union { struct { unsigned short pn_15_0; unsigned int pn_47_16; }
__attribute__((packed));
struct { unsigned short pn_15_0; unsigned int pn_47_16; }
__attribute__((packed)) pn;
};
} __attribute__((packed));
struct wil_ring_dma_addr {
unsigned int addr_low;
unsigned short addr_high;
} __attribute__((packed));
struct vring_tx_dma {
unsigned int d0;
struct wil_ring_dma_addr addr;
unsigned char ip_length;
unsigned char b11;
unsigned char error;
unsigned char status;
unsigned short length;
} __attribute__((packed));
struct vring_tx_desc {
struct vring_tx_mac mac;
struct vring_tx_dma dma;
} __attribute__((packed));
struct wil_ring_tx_enhanced_mac {
unsigned int d[3];
unsigned short tso_mss;
unsigned short scratchpad;
} __attribute__((packed));
struct wil_ring_tx_enhanced_dma {
unsigned char l4_hdr_len;
unsigned char cmd;
unsigned short w1;
struct wil_ring_dma_addr addr;
unsigned char ip_length;
unsigned char b11;
unsigned short addr_high_high;
unsigned short length;
} __attribute__((packed));
struct wil_tx_enhanced_desc {
struct wil_ring_tx_enhanced_mac mac;
struct wil_ring_tx_enhanced_dma dma;
} __attribute__((packed));
union wil_tx_desc {
struct vring_tx_desc legacy;
struct wil_tx_enhanced_desc enhanced;
} __attribute__((packed));
struct vring_rx_dma {
unsigned int d0;
struct wil_ring_dma_addr addr;
unsigned char ip_length;
unsigned char b11;
unsigned char error;
unsigned char status;
unsigned short length;
} __attribute__((packed));
struct vring_rx_desc {
struct vring_rx_mac mac;
struct vring_rx_dma dma;
} __attribute__((packed));
struct wil_ring_rx_enhanced_mac {
unsigned int d[3];
unsigned short buff_id;
unsigned short reserved;
} __attribute((packed));
struct wil_ring_rx_enhanced_dma {
unsigned int d0;
struct wil_ring_dma_addr addr;
unsigned short w5;
unsigned short addr_high_high;
unsigned short length;
} __attribute((packed));
struct wil_rx_enhanced_desc {
struct wil_ring_rx_enhanced_mac mac;
struct wil_ring_rx_enhanced_dma dma;
} __attribute((packed));
union wil_rx_desc {
struct vring_rx_desc legacy;
struct wil_rx_enhanced_desc enhanced;
} __attribute__((packed));
union wil_ring_desc {
union wil_tx_desc tx;
union wil_rx_desc rx;
} __attribute__((packed));
int f (void) {
return sizeof(union wil_ring_desc);
}
$ arm-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=arm-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/arm-linux-gnueabi/13/lto-wrapper
Target: arm-linux-gnueabi
Configured with: ../gcc-13.2.1-20230728/configure --bindir=/usr/bin --build=x86_64-redhat-linux-gnu --datadir=/usr/share --disable-decimal-float --disable-dependency-tracking --disable-gold
--disable-libgcj --disable-libgomp --disable-libmpx --disable-libquadmath --disable-libssp --disable-libunwind-exceptions --disable-shared --disable-silent-rules --disable-sjlj-exceptions
--disable-threads --with-ld=/usr/bin/arm-linux-gnu-ld --enable-__cxa_atexit --enable-checking=release --enable-gnu-unique-object --enable-initfini-array --enable-languages=c,c++
--enable-linker-build-id --enable-lto --enable-nls --enable-obsolete --enable-plugin --enable-targets=all --exec-prefix=/usr --host=x86_64-redhat-linux-gnu --includedir=/usr/include
--infodir=/usr/share/info --libexecdir=/usr/libexec --localstatedir=/var --mandir=/usr/share/man --prefix=/usr --program-prefix=arm-linux-gnu- --sbindir=/usr/sbin --sharedstatedir=/var/lib
--sysconfdir=/etc --target=arm-linux-gnueabi --with-bugurl=http://bugzilla.redhat.com/bugzilla/ --with-gcc-major-version-only --with-isl --with-newlib --with-plugin-ld=/usr/bin/arm-linux-gnu-ld
--with-sysroot=/usr/arm-linux-gnu/sys-root --with-system-libunwind --with-system-zlib --without-headers --with-tune=generic-armv7-a --with-arch=armv7-a --with-float=hard --with-fpu=vfpv3-d16
--with-abi=aapcs-linux --enable-gnu-indirect-function --with-linker-hash-style=gnu
Thread model: single
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.1 20230728 (Red Hat Cross 13.2.1-1) (GCC)
$ arm-linux-gnu-gcc -Os -c t-build-bug.c
$ arm-linux-gnu-objdump -j .text -D t-build-bug.o
t-build-bug.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <f>:
0: e3a00020 mov r0, #32 ;; As expected
4: e12fff1e bx lr
;; Now with a lot of weird things taken from the kernel's CFLAGS...
$ arm-linux-gnu-gcc -mlittle-endian -std=gnu11 -fshort-wchar -funsigned-char -fno-common -fno-PIE -fno-strict-aliasing -fno-dwarf2-cfi-asm -mno-fdpic -fno-omit-frame-pointer -mapcs -mno-sched-prolog
-fno-ipa-sra -mabi=apcs-gnu -mno-thumb-interwork -marm -Wa,-mno-warn-deprecated -march=armv4 -mtune=xscale -msoft-float -Uarm -fno-delete-null-pointer-checks -Os -fno-allow-store-data-races
-fno-reorder-blocks -fno-ipa-cp-clone -fno-partial-inlining -fno-stack-protector -fno-omit-frame-pointer -fno-optimize-sibling-calls -ftrivial-auto-var-init=zero -fno-stack-clash-protection -pg
-fno-inline-functions-called-once -fstrict-flex-arrays=3 -fno-strict-overflow -fno-stack-check -fconserve-stack -fsanitize=kernel-address -fasan-shadow-offset=0x1f000000 --param asan-globals=1 --param
asan-instrumentation-with-call-threshold=10000 --param asan-instrument-allocas=1 --param asan-stack=1 --param asan-kernel-mem-intrinsic-prefix=1 -c t-build-bug.c
$ arm-linux-gnu-objdump -j .text -D t-build-bug.o
t-build-bug.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <f>:
0: e1a0c00d mov ip, sp
4: e92dd800 push {fp, ip, lr, pc}
8: e24cb004 sub fp, ip, #4
c: e52de004 push {lr} @ (str lr, [sp, #-4]!)
10: ebfffffe bl 0 <__gnu_mcount_nc>
14: e3a00022 mov r0, #34 @ 0x22 ;; Hmmm...
sizeof(xxx) changed?
18: e89da800 ldm sp, {fp, sp, pc}
Dmitry
