On Thu, Aug 24, 2017 at 9:58 AM, Christoph Hellwig [off-list ref] wrote:

On Wed, Aug 23, 2017 at 04:48:40PM -0700, Dan Williams wrote:

quoted

We are running running short of vma->vm_flags. We can avoid needing a
new VM_* flag in some cases if the original @flags submitted to mmap(2)
is made available to the ->mmap() 'struct file_operations'
implementation. For example, the proposed addition of MAP_DIRECT can be
implemented without taking up a new vm_flags bit. Another motivation to
avoid vm_flags is that they appear in /proc/$pid/smaps, and we have seen
software that tries to dangerously (TOCTOU) read smaps to infer the
behavior of a virtual address range.

This conversion was performed by the following semantic patch. There
were a few manual edits for oddities like proc_reg_mmap.

Thanks to Julia for helping me with coccinelle iteration to cover cases
where the mmap routine is defined in a separate file from the 'struct
file_operations' instance that consumes it.

How are we going to check that an instance actually supports any
of those flags?

In patch 3 I validate the flags by introducing an
"mmap_supported_mask" field to 'struct file_operations'. It will be
zero by default for almost all implementations and zero means "support
the legacy mmap flags".
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