MAP_FIXED is necessary for almost all execve() and dlopen()

MAP_FIXED is necessary for the proper mapping by execve() of any native ET_EXEC file. The addresses are fixed, after all. There are ET_EXEC files which are several percent smaller and run several percent faster than the corresponding ET_DYN. Some environments have done a good risk-vs-benefit analysis, and for them ET_EXEC is worth it.

MAP_FIXED is necessary for the proper mapping by execve() and dlopen() of any native ET_DYN file which has more than one PT_LOAD, which is nearly all of them. The second and subsequent PT_LOADs must have fixed offsets from the first PT_LOAD. In most cases the best strategy is to use mmap(0, size, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, FD_ANON, 0) to obtain enough contiguous space to hold the convex hull of all PT_LOAD, then mmap() each PT_LOAD into that space using MAP_FIXED.

MAP_FIXED is essential for program-manipulating programs such as valgrind and upx.