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SubjectRe: JIT emulator needs
> There is an SE Linux execmem restriction that enforces W^X.

This depends on whatever SELinux rulesets you are running. Its just a
good rule to have present that most programs shouldn't be self patching,
and then label those that do differently.

> Sometimes it is very helpful to have the read/write mapping
> be a fixed offset from the read/exec mapping. A power of 2
> can be especially desirable.

mmap MAP_FIXED can do this but you need to know a lot about the memory
layout of the system so it gets a bit platform specific.

> Emulators often need a cheap way to change page permissions.

mprotect(, range) rather than a page at a time. The kernel will do

> a. sysctl to set IPC_RMID by default
> b. shmget() flag to set IPC_RMID by default

Use POSIX shared memory

> c. open() flag to unlink a file before returning the fd

Is it really that costly to create a blank file, why do you need to do it
a lot in a JIT ?

> e. mremap() flag to get a read/write mapping of a read/exec one
> f. mremap() flag to get a read/exec mapping of a read/write one
> g. mremap() flag to make the 5th arg (new addr) be the upper limit

This is all mprotect and munmap.

> h. 6-bit wide mremap() "flag" to set the upper limit above given base
> i. support the prot argument to remap_file_pages
> j. a documented way (madvise?) to punch same-VMA zero-page holes

mmap (although you get more VMAs from that) so memset() is probably
genuinely cheaper if the permissions are not changing.

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