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102 lines
4.0 KiB
102 lines
4.0 KiB
2 years ago
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commit c43c5796121bc5bcc0867f02e5536874aa8196c1
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Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
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Date: Wed Dec 30 11:54:00 2020 +0530
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Introduce _FORTIFY_SOURCE=3
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Introduce a new _FORTIFY_SOURCE level of 3 to enable additional
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fortifications that may have a noticeable performance impact, allowing
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more fortification coverage at the cost of some performance.
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With llvm 9.0 or later, this will replace the use of
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__builtin_object_size with __builtin_dynamic_object_size.
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__builtin_dynamic_object_size
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-----------------------------
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__builtin_dynamic_object_size is an LLVM builtin that is similar to
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__builtin_object_size. In addition to what __builtin_object_size
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does, i.e. replace the builtin call with a constant object size,
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__builtin_dynamic_object_size will replace the call site with an
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expression that evaluates to the object size, thus expanding its
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applicability. In practice, __builtin_dynamic_object_size evaluates
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these expressions through malloc/calloc calls that it can associate
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with the object being evaluated.
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A simple motivating example is below; -D_FORTIFY_SOURCE=2 would miss
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this and emit memcpy, but -D_FORTIFY_SOURCE=3 with the help of
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__builtin_dynamic_object_size is able to emit __memcpy_chk with the
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allocation size expression passed into the function:
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void *copy_obj (const void *src, size_t alloc, size_t copysize)
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{
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void *obj = malloc (alloc);
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memcpy (obj, src, copysize);
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return obj;
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}
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Limitations
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-----------
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If the object was allocated elsewhere that the compiler cannot see, or
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if it was allocated in the function with a function that the compiler
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does not recognize as an allocator then __builtin_dynamic_object_size
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also returns -1.
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Further, the expression used to compute object size may be non-trivial
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and may potentially incur a noticeable performance impact. These
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fortifications are hence enabled at a new _FORTIFY_SOURCE level to
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allow developers to make a choice on the tradeoff according to their
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environment.
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diff --git a/include/features.h b/include/features.h
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index ea7673ee115bcf0a..fe9fe16d034fad1b 100644
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--- a/include/features.h
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+++ b/include/features.h
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@@ -381,6 +381,11 @@
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# warning _FORTIFY_SOURCE requires compiling with optimization (-O)
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# elif !__GNUC_PREREQ (4, 1)
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# warning _FORTIFY_SOURCE requires GCC 4.1 or later
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+# elif _FORTIFY_SOURCE > 2 && __glibc_clang_prereq (9, 0)
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+# if _FORTIFY_SOURCE > 3
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+# warning _FORTIFY_SOURCE > 3 is treated like 3 on this platform
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+# endif
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+# define __USE_FORTIFY_LEVEL 3
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# elif _FORTIFY_SOURCE > 1
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# if _FORTIFY_SOURCE > 2
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# warning _FORTIFY_SOURCE > 2 is treated like 2 on this platform
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diff --git a/manual/creature.texi b/manual/creature.texi
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index 8876b2ab779c988f..64f361f27a7d6cdf 100644
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--- a/manual/creature.texi
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+++ b/manual/creature.texi
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@@ -247,7 +247,8 @@ included.
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@standards{GNU, (none)}
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If this macro is defined to @math{1}, security hardening is added to
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various library functions. If defined to @math{2}, even stricter
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-checks are applied.
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+checks are applied. If defined to @math{3}, @theglibc{} may also use
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+checks that may have an additional performance overhead.
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@end defvr
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@defvr Macro _REENTRANT
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diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
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index 3f6fe3cc8563b493..1e39307b0ebcf38f 100644
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--- a/misc/sys/cdefs.h
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+++ b/misc/sys/cdefs.h
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@@ -123,6 +123,15 @@
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#define __bos(ptr) __builtin_object_size (ptr, __USE_FORTIFY_LEVEL > 1)
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#define __bos0(ptr) __builtin_object_size (ptr, 0)
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+/* Use __builtin_dynamic_object_size at _FORTIFY_SOURCE=3 when available. */
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+#if __USE_FORTIFY_LEVEL == 3 && __glibc_clang_prereq (9, 0)
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+# define __glibc_objsize0(__o) __builtin_dynamic_object_size (__o, 0)
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+# define __glibc_objsize(__o) __builtin_dynamic_object_size (__o, 1)
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+#else
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+# define __glibc_objsize0(__o) __bos0 (__o)
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+# define __glibc_objsize(__o) __bos (__o)
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+#endif
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+
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#if __GNUC_PREREQ (4,3)
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# define __warndecl(name, msg) \
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extern void name (void) __attribute__((__warning__ (msg)))
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