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Date: Sat, 10 Jun 2017 12:51:51 +0200
Subject: a possible need for MAP_FIXED in ldso/dynlink.c ?


Running musl-based (1.1.16) Linux binaries (i386) under Linux ABI
on FreeBSD (11.0-RELEASE amd64), with explicit use of the loader like
 /..../ --library-path <something> <prog> <args>
fails when mmap() returns a different address than requested
which is rejected by the musl loader when mapping the executable:
"Not a valid dynamic program",
due to:
        map = ....
                : mmap((void *)addr_min, map_len, prot,
                        MAP_PRIVATE, fd, off_start);
        /* If the loaded file is not relocatable and the requested address is
         * not available, then the load operation must fail. */
        if (eh->e_type != ET_DYN && addr_min && map!=(void *)addr_min) {
                errno = EBUSY;
                goto error;

mmap() returning a different address does not necessarily mean that
the requested one is not available.

I wonder whether adding MAP_FIXED to MAP_PRIVATE above would be a
useful approach (conditionally on eh->e_type==ET_EXEC ?).

Adding the MAP_FIXED flag, both conditionally or not, seems to work
around the particular problem but I am unsure about all its implications
and consequences, among others under the current Linux implementation
of the Lunux ABI.


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