Remove some compatability with Seventh Edition UNIX realloc().

[brooks]

Summary

In Seventh Edition UNIX, the last pointer passed to free() was
guaranteed to not actually have been freed allowing memory to be
"compacted" via the following pattern:

free(foo);
foo = realloc(foo, newsize);

Further, Andrew Koenig reports in "C Traps and Pitfalls" that the
original realloc() implementation required this pattern.

The C standard is clear that this is Undefined Behavior. Modern
allocators don't support it and no portable code could rely on it so
remove this support.

Note: the removed implementation contains an off-by-one error and if
an item isn't found on the freelist, then twice as much memory as the
largest possible allocation will be copied.

[imp]

This is good, and good commit message. V7 was released 40 years ago this past January. It flourished in the late 70s and early 80s as a platform people ported to (I'm aware of maybe a dozen such ports too 4 or 5 different architectures)... then all those ports migrated to System III or System V by maybe 1989 or so. The last time I saw this software pattern was in the mid 90s when I updated the program in question to not do that.... I doubt this code path has been execute in the last decade or two except by pedants testing V7 compatibility.

[pstef]

libexec/rtld-elf/rtld_malloc.c

318 ↗

(On Diff #60921)

Is this comment (rtld_malloc.c:318) talking about the same compacting as you mentioned in the commit message?

[kib]

libexec/rtld-elf/rtld_malloc.c

257 ↗

(On Diff #60921)

This var should be removed ...

273 ↗

(On Diff #60921)

... and this condition as well, executing the code under it unconditionally.

288 ↗

(On Diff #60921)

Note that cp is already freed at this point. If you claim that __crt_realloc() does not support/does not rely on accessibility of the block freed just before, then this copy should be done before __crt_free(cp).

[brooks]

• Always follow the allocated path, don't free prematurely.

[kib]

Please fix two style issues, no need to re-post the updated patch.

libexec/rtld-elf/rtld_malloc.c

264 ↗

(On Diff #60989)

I would express this as

if (op->ov_magic != MAGIC)
   return (NULL);
i = op->ov_index;

I considered should we change the if (... != MAGIC) to assert, esp. because the only consumers of this malloc are rtld and libthr, but decided that we do not.

271 ↗

(On Diff #60989)

if (i != 0) {