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authorLinus Torvalds <torvalds@linux-foundation.org>2017-05-11 21:27:54 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2017-05-11 21:27:54 +0300
commite47b40a235e9552bc48e4c73c53854ce734da661 (patch)
tree6cd1889e9ba1b0607326917a70eb89b0008b8067 /Documentation
parent4892c6f72e4b335b162edadbefc5378cea4f0ee1 (diff)
parent0c2cf6d9487cb90be6ad7fac66044dfa8e8e5243 (diff)
downloadlinux-e47b40a235e9552bc48e4c73c53854ce734da661.tar.xz
Merge tag 'arm64-upstream' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux
Pull more arm64 updates from Catalin Marinas: - Silence module allocation failures when CONFIG_ARM*_MODULE_PLTS is enabled. This requires a check for __GFP_NOWARN in alloc_vmap_area() - Improve/sanitise user tagged pointers handling in the kernel - Inline asm fixes/cleanups * tag 'arm64-upstream' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux: arm64: Silence first allocation with CONFIG_ARM64_MODULE_PLTS=y ARM: Silence first allocation with CONFIG_ARM_MODULE_PLTS=y mm: Silence vmap() allocation failures based on caller gfp_flags arm64: uaccess: suppress spurious clang warning arm64: atomic_lse: match asm register sizes arm64: armv8_deprecated: ensure extension of addr arm64: uaccess: ensure extension of access_ok() addr arm64: ensure extension of smp_store_release value arm64: xchg: hazard against entire exchange variable arm64: documentation: document tagged pointer stack constraints arm64: entry: improve data abort handling of tagged pointers arm64: hw_breakpoint: fix watchpoint matching for tagged pointers arm64: traps: fix userspace cache maintenance emulation on a tagged pointer
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/arm64/tagged-pointers.txt62
1 files changed, 47 insertions, 15 deletions
diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt
index d9995f1f51b3..a25a99e82bb1 100644
--- a/Documentation/arm64/tagged-pointers.txt
+++ b/Documentation/arm64/tagged-pointers.txt
@@ -11,24 +11,56 @@ in AArch64 Linux.
The kernel configures the translation tables so that translations made
via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
the virtual address ignored by the translation hardware. This frees up
-this byte for application use, with the following caveats:
+this byte for application use.
- (1) The kernel requires that all user addresses passed to EL1
- are tagged with tag 0x00. This means that any syscall
- parameters containing user virtual addresses *must* have
- their top byte cleared before trapping to the kernel.
- (2) Non-zero tags are not preserved when delivering signals.
- This means that signal handlers in applications making use
- of tags cannot rely on the tag information for user virtual
- addresses being maintained for fields inside siginfo_t.
- One exception to this rule is for signals raised in response
- to watchpoint debug exceptions, where the tag information
- will be preserved.
+Passing tagged addresses to the kernel
+--------------------------------------
- (3) Special care should be taken when using tagged pointers,
- since it is likely that C compilers will not hazard two
- virtual addresses differing only in the upper byte.
+All interpretation of userspace memory addresses by the kernel assumes
+an address tag of 0x00.
+
+This includes, but is not limited to, addresses found in:
+
+ - pointer arguments to system calls, including pointers in structures
+ passed to system calls,
+
+ - the stack pointer (sp), e.g. when interpreting it to deliver a
+ signal,
+
+ - the frame pointer (x29) and frame records, e.g. when interpreting
+ them to generate a backtrace or call graph.
+
+Using non-zero address tags in any of these locations may result in an
+error code being returned, a (fatal) signal being raised, or other modes
+of failure.
+
+For these reasons, passing non-zero address tags to the kernel via
+system calls is forbidden, and using a non-zero address tag for sp is
+strongly discouraged.
+
+Programs maintaining a frame pointer and frame records that use non-zero
+address tags may suffer impaired or inaccurate debug and profiling
+visibility.
+
+
+Preserving tags
+---------------
+
+Non-zero tags are not preserved when delivering signals. This means that
+signal handlers in applications making use of tags cannot rely on the
+tag information for user virtual addresses being maintained for fields
+inside siginfo_t. One exception to this rule is for signals raised in
+response to watchpoint debug exceptions, where the tag information will
+be preserved.
The architecture prevents the use of a tagged PC, so the upper byte will
be set to a sign-extension of bit 55 on exception return.
+
+
+Other considerations
+--------------------
+
+Special care should be taken when using tagged pointers, since it is
+likely that C compilers will not hazard two virtual addresses differing
+only in the upper byte.