Alignment trap seems unable to handle ARM VLDR instruction #3099
Description
Describe the bug
I run into SIGBUS crashes due to unaligned memory access via the VLDR instruction in one of my applications running on a raspberry pi. The application in question uses zero-copy networking, which leads to unaligned memory. Specifically, I have a few float values which are not 4 byte aligned. The kernel seems to be fine with using the LDR instruction on unaligned memory, however when the compiler decides to rely on the VLDR instruction, SIGBUS is thrown.
To reproduce
I managed to generate a small code example which exactly reproduces what I am seeing. See the code below and compile without optimization. In the additional context you see the generated assembly code for foo::x() and foo::x_id() on my machine. In the first case, the compiler relies on LDR and VMOV, whereas the second function uses VLDR and causes a SIGBUS error. See the excerpt from the journal.
#include <cstring>
#include <iostream>
float ident(float in) { return in; }
struct foo final {
foo(float _x) : x_(_x) {}
float x() const { return x_; }
float x_id() const { return ident(x_); }
float x_;
};
int main(int argc, char const *argv[]) {
char *buffer = new char[10];
foo orig(.2);
foo *copy = (foo *)(buffer + 2);
std::memcpy(copy, &orig, sizeof(foo));
std::cout << "x: " << copy->x() << std::endl;
std::cout << "x_id: " << copy->x_id() << std::endl;
delete[] buffer;
return 0;
}
Expected behaviour
I'm not entirely sure whether this is a kernel bug and the kernel should be able to handle this unaligned memory access. Based on the ARM manual the The comments in the respective kernel code do not talk about the VLDR instruction should be able to handle an unaligned memory address, so the alignment trap shouldn't even trigger?VLDR instruction.
Actual behaviour
The process crashes with SIGBUS.
System
Copy and paste the results of the raspinfo command in to this section. Alternatively, copy and paste a pastebin link, or add answers to the following questions:
- gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/arm-linux-gnueabihf/8/lto-wrapper
Target: arm-linux-gnueabihf
Configured with: ../src/configure -v --with-pkgversion='Raspbian 8.3.0-6+rpi1' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-8 --program-prefix=arm-linux-gnueabihf- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libitm --disable-libquadmath --disable-libquadmath-support --enable-plugin --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-sjlj-exceptions --with-arch=armv6 --with-fpu=vfp --with-float=hard --disable-werror --enable-checking=release --build=arm-linux-gnueabihf --host=arm-linux-gnueabihf --target=arm-linux-gnueabihf
Thread model: posix
gcc version 8.3.0 (Raspbian 8.3.0-6+rpi1)
- Which model of Raspberry Pi?
Pi3B - Which OS and version (
cat /etc/rpi-issue)?
Raspberry Pi reference 2019-07-10
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 175dfb027ffabd4b8d5080097af0e51ed9a4a56c, stage2
- Which firmware version (
vcgencmd version)?
Jul 9 2019 14:40:53
Copyright (c) 2012 Broadcom
version 6c3fe3f096a93de3b34252ad98cdccadeb534be2 (clean) (release) (start)
- Which kernel version (
uname -a)?
Linux raspberrypi 4.19.57-v7+ #1244 SMP Thu Jul 4 18:45:25 BST 2019 armv7l GNU/Linux
Logs
Jul 23 17:00:45 raspberrypi kernel: Alignment trap: a.out (1119) PC=0x0001098c Instr=0xedd37a00 Address=0x01cd005a FSR 0x001
Jul 23 17:00:45 raspberrypi kernel: Alignment trap: not handling instruction edd37a00 at [<0001098c>]
Jul 23 17:00:45 raspberrypi kernel: Unhandled fault: alignment exception (0x001) at 0x01cd005a
Jul 23 17:00:45 raspberrypi kernel: pgd = 3e055f1c
Jul 23 17:00:45 raspberrypi kernel: [01cd005a] *pgd=15bf3835, *pte=1322d75f, *ppte=1322dc7f
Additional context
Alignment trap is setup to warn+fixup
pi@raspberrypi:~ $ cat /proc/cpu/alignment
User: 7
System: 0 ( (null))
Skipped: 7
Half: 0
Word: 0
DWord: 0
Multi: 0
User faults: 3 (fixup+warn)
0001094c <foo::x() const>:
1094c: e52db004 push {fp} ; (str fp, [sp, #-4]!)
10950: e28db000 add fp, sp, #0
10954: e24dd00c sub sp, sp, #12
10958: e50b0008 str r0, [fp, #-8]
1095c: e51b3008 ldr r3, [fp, #-8]
10960: e5933000 ldr r3, [r3]
10964: ee073a90 vmov s15, r3
10968: eeb00a67 vmov.f32 s0, s15
1096c: e28bd000 add sp, fp, #0
10970: e49db004 pop {fp} ; (ldr fp, [sp], #4)
10974: e12fff1e bx lr
00010978 <foo::x_id() const>:
10978: e92d4800 push {fp, lr}
1097c: e28db004 add fp, sp, #4
10980: e24dd008 sub sp, sp, #8
10984: e50b0008 str r0, [fp, #-8]
10988: e51b3008 ldr r3, [fp, #-8]
1098c: edd37a00 vldr s15, [r3]
10990: eeb00a67 vmov.f32 s0, s15
10994: ebffff78 bl 1077c <ident(float)>
10998: eef07a40 vmov.f32 s15, s0
1099c: eeb00a67 vmov.f32 s0, s15
109a0: e24bd004 sub sp, fp, #4
109a4: e8bd8800 pop {fp, pc}