On Thu, Apr 19, 2018 at 5:30 PM, Zack Weinberg zackw@panix.com wrote:
On Thu, Apr 19, 2018 at 10:37 AM, Arnd Bergmann arnd@arndb.de wrote:
Most architectures now use the asm-generic copy of the sysvipc data structures (msqid64_ds, semid64_ds, shmid64_ds), which use 32-bit __kernel_time_t on 32-bit architectures but have padding behind them to allow extending the type to 64-bit.
Unfortunately, that fails on all big-endian architectures, which have the padding on the wrong side. As so many of them get it wrong, we decided to not bother even trying to fix it up when we introduced the asm-generic copy. Instead we always use the padding word now to provide the upper 32 bits of the seconds value, regardless of the endianess.
A libc implementation on a typical big-endian system can deal with this by providing its own copy of the structure definition to user space, and swapping the two 32-bit words before returning from the semctl/shmctl/msgctl system calls.
This seems generally like a sound approach, but I need to ask whether any of the structures involved can ever appear in a sendmsg() control message (that is, in the data pointed to by msg_control), or an AF_NETLINK message, or any other situation where the kernel communicates a structured message of arbitrary size to user space or vice versa. libc can't munge those messages, because new message types can be added faster than libc can keep up with them, and because I/O primitives like sendmsg() generally aren't allowed to allocate arbitrarily-large scratch buffers.
I'm fairly sure that the sysvipc data structures are entirely distinct from the structures that get passed over sockets, so the question of socket data is unrelated to this series and will be addressed in a separate series.
To give some background on what needs to be done for sockets, the only incompatibility I'm aware of are socket timestamps that get enabled with SO_TIMESTAMP, SO_TIMESTAMPNS or SO_TIMESTAMPING and get passed from kernel to user space as SCM_TIMESTAMP/SCM_TIMESTAMPNS/SCM_TIMESTAMPING cmsg data.
We already have code for handling 32-bit compat applications on 64-bit kernels, but that cannot work for 32-bit applications if the kernel has no idea whether the application uses 32-bit or 64-bit time_t, and we don't have a function like in_compat_syscall() that we can use to find that out.
Our plan here is to change asm/socket.h to have three additional timestamp flags that correspond to the existing SO_TIMESTAMP* flags but signify that user space expects the new structure layout (which is compatible with the existing layout on 64-bit kernels). For each flag, the kernel then defines a wrapper that (on 32-bit user space) looks like
#define SO_TIMESTAMP (sizeof(time_t) > sizeof(__kernel_long_t) ? \ SO_TIMESTAMP_TIME64 : SO_TIMESTAMP_OLD)
Any application asking for SO_TIMESTAMP_OLD will get the traditional behavior, while applications that are built with a 64-bit time_t will pass SO_TIMESTAMP_TIME64 into setsockopts, causing the kernel to use the new behavior. In 64-bit tasks, we probably want to define both to have existing behavior even though one would never see the new macro.
Arnd