fortify: Hide run-time copy size from value range tracking

GCC performs value range tracking for variables as a way to provide better
diagnostics. One place this is regularly seen is with warnings associated
with bounds-checking, e.g. -Wstringop-overflow, -Wstringop-overread,
-Warray-bounds, etc. In order to keep the signal-to-noise ratio high,
warnings aren't emitted when a value range spans the entire value range
representable by a given variable. For example:

	unsigned int len;
	char dst[8];
	...
	memcpy(dst, src, len);

If len's value is unknown, it has the full "unsigned int" range of [0,
UINT_MAX], and GCC's compile-time bounds checks against memcpy() will
be ignored. However, when a code path has been able to narrow the range:

	if (len > 16)
		return;
	memcpy(dst, src, len);

Then the range will be updated for the execution path. Above, len is
now [0, 16] when reading memcpy(), so depending on other optimizations,
we might see a -Wstringop-overflow warning like:

	error: '__builtin_memcpy' writing between 9 and 16 bytes into region of size 8 [-Werror=stringop-overflow]

When building with CONFIG_FORTIFY_SOURCE, the fortified run-time bounds
checking can appear to narrow value ranges of lengths for memcpy(),
depending on how the compiler constructs the execution paths during
optimization passes, due to the checks against the field sizes. For
example:

	if (p_size_field != SIZE_MAX &&
	    p_size != p_size_field && p_size_field < size)

As intentionally designed, these checks only affect the kernel warnings
emitted at run-time and do not block the potentially overflowing memcpy(),
so GCC thinks it needs to produce a warning about the resulting value
range that might be reaching the memcpy().

We have seen this manifest a few times now, with the most recent being
with cpumasks:

In function ‘bitmap_copy’,
    inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2,
    inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2:
./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread]
  114 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’
  633 |         __underlying_##op(p, q, __fortify_size);                        \
      |         ^~~~~~~~~~~~~
./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’
  678 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’
  259 |                 memcpy(dst, src, len);
      |                 ^~~~~~
kernel/padata.c: In function ‘__padata_set_cpumasks’:
kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256]
  713 |                                  cpumask_var_t pcpumask,
      |                                  ~~~~~~~~~~~~~~^~~~~~~~

This warning is _not_ emitted when CONFIG_FORTIFY_SOURCE is disabled,
and with the recent -fdiagnostics-details we can confirm the origin of
the warning is due to FORTIFY's bounds checking:

../include/linux/bitmap.h:259:17: note: in expansion of macro 'memcpy'
  259 |                 memcpy(dst, src, len);
      |                 ^~~~~~
  '__padata_set_cpumasks': events 1-2
../include/linux/fortify-string.h:613:36:
  612 |         if (p_size_field != SIZE_MAX &&
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  613 |             p_size != p_size_field && p_size_field < size)
      |             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
      |                                    |
      |                                    (1) when the condition is evaluated to false
      |                                    (2) when the condition is evaluated to true
  '__padata_set_cpumasks': event 3
  114 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
      |                                 |
      |                                 (3) out of array bounds here

Note that the cpumask warning started appearing since bitmap functions
were recently marked __always_inline in commit ed8cd2b3bd ("bitmap:
Switch from inline to __always_inline"), which allowed GCC to gain
visibility into the variables as they passed through the FORTIFY
implementation.

In order to silence these false positives but keep otherwise deterministic
compile-time warnings intact, hide the length variable from GCC with
OPTIMIZE_HIDE_VAR() before calling the builtin memcpy.

Additionally add a comment about why all the macro args have copies with
const storage.

Reported-by: "Thomas Weißschuh" <linux@weissschuh.net>
Closes: https://lore.kernel.org/all/db7190c8-d17f-4a0d-bc2f-5903c79f36c2@t-8ch.de/
Reported-by: Nilay Shroff <nilay@linux.ibm.com>
Closes: https://lore.kernel.org/all/20241112124127.1666300-1-nilay@linux.ibm.com/
Tested-by: Nilay Shroff <nilay@linux.ibm.com>
Acked-by: Yury Norov <yury.norov@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kees Cook <kees@kernel.org>

include/linux/fortify-string.h

@@ -616,6 +616,12 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
 	return false;
 }
 
+/*
+ * To work around what seems to be an optimizer bug, the macro arguments
+ * need to have const copies or the values end up changed by the time they
+ * reach fortify_warn_once(). See commit 6f7630b1b5bc ("fortify: Capture
+ * __bos() results in const temp vars") for more details.
+ */
 #define __fortify_memcpy_chk(p, q, size, p_size, q_size,		\
 			     p_size_field, q_size_field, op) ({		\
 	const size_t __fortify_size = (size_t)(size);			\
@@ -623,6 +629,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
 	const size_t __q_size = (q_size);				\
 	const size_t __p_size_field = (p_size_field);			\
 	const size_t __q_size_field = (q_size_field);			\
+	/* Keep a mutable version of the size for the final copy. */	\
+	size_t __copy_size = __fortify_size;				\
 	fortify_warn_once(fortify_memcpy_chk(__fortify_size, __p_size,	\
 				     __q_size, __p_size_field,		\
 				     __q_size_field, FORTIFY_FUNC_ ##op), \
@@ -630,7 +638,11 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
 		  __fortify_size,					\
 		  "field \"" #p "\" at " FILE_LINE,			\
 		  __p_size_field);					\
-	__underlying_##op(p, q, __fortify_size);			\
+	/* Hide only the run-time size from value range tracking to */	\
+	/* silence compile-time false positive bounds warnings. */	\
+	if (!__builtin_constant_p(__copy_size))				\
+		OPTIMIZER_HIDE_VAR(__copy_size);			\
+	__underlying_##op(p, q, __copy_size);				\
 })
 
 /*