linux-user: Fix loading of BSS segments

linux-user: Fix loading of BSS segments

Some ELF binaries encode the .bss section as an extension of the data
ones by setting the segment p_memsz > p_filesz. Some other binaries take
a different route and encode it as a stand-alone PT_LOAD segment with
p_filesz = 0 and p_memsz > 0.

Both the encodings are actually correct per ELF specification but the
ELF loader had some troubles in handling the former: with the old logic
it was very likely to get Qemu to crash in zero_bss when trying to
access unmapped memory.

zero_bss isn't meant to allocate whole zero-filled segments but to
"complete" a previously mapped segment with the needed zero bits.

The fix is pretty simple, if the segment is completely zero-filled we
simply allocate one or more pages (according to p_memsz) and avoid
calling zero_bss altogether.

Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
Message-Id: <c9106487-dc4d-120a-bd48-665b3c617287@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>