malloc: make malloc fail with requests larger than PTRDIFF_MAX (BZ#23741)

As discussed previously on libc-alpha [1], this patch follows up the idea
and add both the __attribute_alloc_size__ on malloc functions (malloc,
calloc, realloc, reallocarray, valloc, pvalloc, and memalign) and limit
maximum requested allocation size to up PTRDIFF_MAX (taking into
consideration internal padding and alignment).

This aligns glibc with gcc expected size defined by default warning
-Walloc-size-larger-than value which warns for allocation larger than
PTRDIFF_MAX.  It also aligns with gcc expectation regarding libc and
expected size, such as described in PR#67999 [2] and previously discussed
ISO C11 issues [3] on libc-alpha.

From the RFC thread [4] and previous discussion, it seems that consensus
is only to limit such requested size for malloc functions, not the system
allocation one (mmap, sbrk, etc.).

The implementation changes checked_request2size to check for both overflow
and maximum object size up to PTRDIFF_MAX. No additional checks are done
on sysmalloc, so it can still issue mmap with values larger than
PTRDIFF_T depending on the requested size.

The __attribute_alloc_size__ is for functions that return a pointer only,
which means it cannot be applied to posix_memalign (see remarks in GCC
PR#87683 [5]). The runtimes checks to limit maximum requested allocation
size does applies to posix_memalign.

Checked on x86_64-linux-gnu and i686-linux-gnu.

[1] https://sourceware.org/ml/libc-alpha/2018-11/msg00223.html
[2] https://gcc.gnu.org/bugzilla//show_bug.cgi?id=67999
[3] https://sourceware.org/ml/libc-alpha/2011-12/msg00066.html
[4] https://sourceware.org/ml/libc-alpha/2018-11/msg00224.html
[5] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87683

	[BZ #23741]
	* malloc/hooks.c (malloc_check, realloc_check): Use
	__builtin_add_overflow on overflow check and adapt to
	checked_request2size change.
	* malloc/malloc.c (__libc_malloc, __libc_realloc, _mid_memalign,
	__libc_pvalloc, __libc_calloc, _int_memalign): Limit maximum
	allocation size to PTRDIFF_MAX.
	(REQUEST_OUT_OF_RANGE): Remove macro.
	(checked_request2size): Change to inline function and limit maximum
	requested size to PTRDIFF_MAX.
	(__libc_malloc, __libc_realloc, _int_malloc, _int_memalign): Limit
	maximum allocation size to PTRDIFF_MAX.
	(_mid_memalign): Use _int_memalign call for overflow check.
	(__libc_pvalloc): Use __builtin_add_overflow on overflow check.
	(__libc_calloc): Use __builtin_mul_overflow for overflow check and
	limit maximum requested size to PTRDIFF_MAX.
	* malloc/malloc.h (malloc, calloc, realloc, reallocarray, memalign,
	valloc, pvalloc): Add __attribute_alloc_size__.
	* stdlib/stdlib.h (malloc, realloc, reallocarray, valloc): Likewise.
	* malloc/tst-malloc-too-large.c (do_test): Add check for allocation
	larger than PTRDIFF_MAX.
	* malloc/tst-memalign.c (do_test): Disable -Walloc-size-larger-than=
	around tests of malloc with negative sizes.
	* malloc/tst-posix_memalign.c (do_test): Likewise.
	* malloc/tst-pvalloc.c (do_test): Likewise.
	* malloc/tst-valloc.c (do_test): Likewise.
	* malloc/tst-reallocarray.c (do_test): Replace call to reallocarray
	with resulting size allocation larger than PTRDIFF_MAX with
	reallocarray_nowarn.
	(reallocarray_nowarn): New function.
	* NEWS: Mention the malloc function semantic change.

1 files changed, 6 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index b58e246..db1ca81 100644
--- a/NEWS
+++ b/NEWS
@@ -160,6 +160,12 @@ Deprecated and removed features, and other changes affecting compatibility:
   as all functions that call vscanf, vfscanf, or vsscanf are annotated with
   __attribute__ ((format (scanf, ...))).
 
+* Memory allocation functions malloc, calloc, realloc, reallocarray, valloc,
+  pvalloc, memalign, and posix_memalign fail now with total object size
+  larger than PTRDIFF_MAX.  This is to avoid potential undefined behavior with
+  pointer subtraction within the allocated object, where results might
+  overflow the ptrdiff_t type.
+
 Changes to build and runtime requirements:
 
 * Python 3.4 or later is required to build the GNU C Library.