Re: [PATCH] manual: Document replacing malloc [BZ #20424]

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 20/04/17 11:53, Florian Weimer wrote:
> +@node Replacing malloc
> +@subsection Replacing @code{malloc}
> +
> +@cindex @code{malloc} replacement
> +@cindex @code{LD_PRELOAD} and @code{malloc}
> +@cindex alternative @code{malloc} implementations
> +@cindex customizing @code{malloc}
> +@cindex interposing @code{malloc}
> +@cindex preempting @code{malloc}
> +@cindex replacing @code{malloc}
> +@Theglibc{} supports replacing the built-in @code{malloc} implementation
> +with a different allocator with the same interface.  For dynamically
> +linked programs, this happens through ELF symbol interposition, either
> +using shared object dependencies or @code{LD_PRELOAD}.  For static
> +linking, the @code{malloc} replacement library must be linked in before
> +linking against @code{libc.a} (explicitly or implicitly).
> +

this documentation does not mention known caveats, e.g.

- when wrapping calloc via dlsym, dlsym may call calloc, the
  user has to deal with it,

- similarly any interface that internally may use malloc (in
  the future) better not be used in the malloc implementation.

- malloc may be called when locks are held (e.g. some stdio
  lock during scanf) so synchronizing with anything that might
  also hold the same lock in the malloc implementation may
  deadlock (a more interesting example is probably the dl_load_lock
  while allocating dynamic tls in some cases so a user provided
  malloc should not use dtls)

- some other invariants may not hold internally in libc when
  malloc is called, so the malloc implementation should not
  rely on those. (e.g. posix tsd deallocation calls free which
  might observe tsd in an inconsistent state)

- glibc tries to provide some guarantees that may not work
  with interposed malloc (unlocking malloc locks at multi-
  threaded fork ?)

it would be useful to specify the libc internal malloc interface
contracts, but that is a lot harder than just listing the set of
interfaces that need to be provided.

> +@strong{Note:} Failure to provide a complete set of replacement
> +functions (that is, all the functions used by the application,
> +@theglibc{}, and other linked-in libraries) can lead to static linking
> +failures, and, at run time, to heap corruption and application crashes.
> +
> +The minimum set of functions which has to be provided by a custom
> +@code{malloc} is given in the table below.
> +
> +@table @code
> +@item malloc
> +@item free
> +@item calloc
> +@item realloc
> +@end table
> +
> +These @code{malloc}-related functions are required for @theglibc{} to
> +work.@footnote{Versions of @theglibc{} before 2.25 required that a
> +custom @code{malloc} defines @code{__libc_memalign} (with the same
> +interface as the @code{memalign} function).}
> +
> +The @code{malloc} implementation in @theglibc{} provides additional
> +functionality not used by the library itself, but which is often used by
> +other system libraries and applications.  A general-purpose replacement
> +@code{malloc} implementation should provide definitions of these
> +functions, too.  Their names are listed in the following table.
> +
> +@table @code
> +@item aligned_alloc
> +@item malloc_usable_size
> +@item memalign
> +@item posix_memalign
> +@item pvalloc
> +@item valloc
> +@end table
> +
> +In addition, very old applications may use the obsolete @code{cfree}
> +function.
> +
> +Further @code{malloc}-related functions such as @code{mallopt} or
> +@code{mallinfo} will not have any effect or return incorrect statistics
> +when a replacement @code{malloc} is in use.  However, failure to replace
> +these functions typically does not result in crashes or other incorrect
> +application behavior, but may result in static linking failures.
> +
>  @node Obstacks
>  @subsection Obstacks
>  @cindex obstacks
>