On Tue, Sep 19, 2023 at 10:02:17AM +0200, Miklos Szeredi wrote: > On Tue, 19 Sept 2023 at 02:38, Matthew House <mattlloydhouse@xxxxxxxxx> wrote: > > > One natural solution is to set either of the two lengths to the expected > > size if the provided buffer are too small. That way, the caller learns both > > which of the buffers is too small, and how large they need to be. Replacing > > a provided size with an expected size in this way already has precedent in > > existing syscalls: > > This is where the thread started. Knowing the size of the buffer is > no good, since the needed buffer could change between calls. The same problem would exist for the single buffer. Realistically, users will most often simply use a fixed size PATH_MAX buffer that will cover most cases and fallback to allocating a larger buffer in case things go awry. I don't think we need to make this atomic either. Providing a hint for the required buffer size in case this fails is good enough and should be a rather rare occurence and is exactly how other variable-sized buffers are handled. > Also having the helper allocate buffers inside the struct could easily > result in leaks since it's not obvious what the caller needs to free, I don't think we need to be overly concerned with how userspace implements the wrapper here. Leaks can occur in both scenarios and low-level userspace can use automatic cleanup macros (we even support it in the kernel since v6.5) to harden against this. Really, the main things I care about are 64 bit alignment of the whole struct, typed __u64 pointers with __u32 size for mnt_root and mnt_point and that we please spell out "mount" and not use "mnt": so statmount because the new mount api uses "mount" (move_mount(), mount_setattr(), fsmount(), MOUNT_ATTR_*) almost everywhere.
