[ARM] unexpected sizeof() of a complex packed type

[Andrew Pinski]

On Thu, Nov 16, 2023 at 8:42 AM Dmitry Antipov <dmantipov@yandex.ru> wrote:
>
> (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}

Well for one -mabi=apcs-gnu and -mabi=aapcs-linux are 2 different ABIs
and have different rules.

The difference is due to the anonymous union:
>         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;
>         };

If we add __attribute__((packed)) at the end of the union, the size is
"corrected".
Since apcs-gnu is considered an ABI pre EABI for GCC I really doubt it
can be changed here.

Thanks,
Andrew

>
> Dmitry