rtld: When relocating, map without PROT_EXEC

[emaste]

Summary

This makes text segment relocation work under W^X.

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I have encountered this when loading lucet-compiled shared objects with W^X (D24933).

[emaste]

libexec/rtld-elf/rtld.c

2986–2987

maybe clearer if we incorporate the PROT_WRITE part from above?

something like

prot = convert_prot(ph->p_flags);
if (before)
        prot = prot & ~PROT_EXEC | PROT_WRITE;

[kib]

libexec/rtld-elf/rtld.c

2986–2987

There is no much reason to use convert_prot() in'before' case at all, then. Only PF_R survives. It can be

prot = before ? (PROT_READ | PROT_WRITE) : convert_prot(ph->p_flags);

[emaste]

Added to my staging tree here: https://github.com/emaste/freebsd/commit/138b3637e80a4ececdc0eaf8aea734262217d0b0

[emaste]

Update per kib's suggestion