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CFI: Add a compiler built-in that returns the actual address of a function #1353

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samitolvanen opened this issue Apr 20, 2021 · 4 comments
Closed

CFI: Add a compiler built-in that returns the actual address of a function #1353

samitolvanen opened this issue Apr 20, 2021 · 4 comments
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@samitolvanen
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[FEATURE] CFI Related to building the kernel with Clang Control Flow Integrity feature-request Not a bug per-se [FIXED][LLVM] 14 This bug was fixed in LLVM 14.x

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@samitolvanen
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With -fno-sanitize-cfi-canonical-jump-tables, the compiler replaces function references with references to the CFI jump table. This is problematic when we want to initialize global variables with actual function addresses, such as in arch/x86/kernel/idt.c. Currently, the only solution is to disable CFI for the entire translation unit. Elsewhere in code, we can alternatively use inline assembly to take the function address (see function_nocfi()), but kernel maintainers aren't too thrilled about that approach either.

Solution: Add a compiler built-in, similar to __builtin_addressof(), to LLVM that always returns a pointer to the actual function. Peter suggests this might require a new constant expression type, similar to DSOLocalEquivalent.

cc @nickdesaulniers @kees @pcc
@samitolvanen samitolvanen added feature-request Not a bug per-se [FEATURE] CFI Related to building the kernel with Clang Control Flow Integrity non-canonical jump tables Issue related to the use of non-canonical jump tables in CFI labels Apr 20, 2021
@kees
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kees commented Apr 20, 2021

Yeah, this would likely clear the way for x86 CFI. Right now, it's a hard blocker. :(
https://lore.kernel.org/lkml/CALCETrWUS52tzLNiWL5sAVVB5-ko1EW73-TEiO=eZ5jF_QyGPQ@mail.gmail.com/

samitolvanen added a commit to samitolvanen/linux that referenced this issue May 20, 2021
@samitolvanen
arm64: pass stack protector flags to the linker with LTO
ced6ed5 
Clang 13 will support CONFIG_STACKPROTECTOR_PER_TASK, but we need to
also pass the command line arguments to the linker to use it together
with CONFIG_LTO_CLANG.

Link: ClangBuiltLinux#1353
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
@kees kees mentioned this issue Jun 4, 2021
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@samitolvanen
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Initial PoC:
https://reviews.llvm.org/D108478
https://reviews.llvm.org/D108479

@nickdesaulniers nickdesaulniers added the [PATCH] Submitted A patch has been submitted for review label Sep 8, 2021
samitolvanen added a commit to samitolvanen/llvm-project that referenced this issue Dec 20, 2021
@samitolvanen
[llvm][IR] Add no_cfi constant
1d8db85 
With Control-Flow Integrity (CFI), the LowerTypeTests pass replaces
function references with CFI jump table references, which is a problem
for low-level code that needs the address of the actual function body.

For example, in the Linux kernel, the code that sets up interrupt
handlers needs to take the address of the interrupt handler function
instead of the CFI jump table, as the jump table may not even be mapped
into memory when an interrupt is triggered.

This change adds the no_cfi constant type, which wraps function
references in a value that LowerTypeTestsModule::replaceCfiUses does not
replace.

Link: ClangBuiltLinux/linux#1353

Reviewed By: nickdesaulniers, pcc

Differential Revision: https://reviews.llvm.org/D108478
samitolvanen added a commit to samitolvanen/llvm-project that referenced this issue Dec 20, 2021
@samitolvanen
[Clang] Add __builtin_function_start
f9b0c69 
Control-Flow Integrity (CFI) replaces references to address-taken
functions with pointers to the CFI jump table. This is a problem
for low-level code, such as operating system kernels, which may
need the address of an actual function body without the jump table
indirection.

This change adds the __builtin_function_start() builtin, which
accepts an argument that can be constant-evaluated to a function,
and returns the address of the function body.

Link: ClangBuiltLinux/linux#1353

Depends on D108478

Reviewed By: pcc, rjmccall

Differential Revision: https://reviews.llvm.org/D108479
@samitolvanen
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Fixed in llvm/llvm-project@5dc8aaa and llvm/llvm-project@ec2e26e.

@samitolvanen samitolvanen added [PATCH] Accepted A submitted patch has been accepted upstream and removed [PATCH] Submitted A patch has been submitted for review non-canonical jump tables Issue related to the use of non-canonical jump tables in CFI labels Dec 21, 2021
@nickdesaulniers nickdesaulniers added [FIXED][LLVM] 14 This bug was fixed in LLVM 14.x and removed [PATCH] Accepted A submitted patch has been accepted upstream labels Dec 21, 2021
@nickdesaulniers
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@samitolvanen sent https://lore.kernel.org/lkml/20220401201916.1487500-1-samitolvanen@google.com/ to use this in the kernel.

samitolvanen added a commit to samitolvanen/linux that referenced this issue Apr 5, 2022
@samitolvanen
cfi: Use __builtin_function_start
1d11633 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
intel-lab-lkp pushed a commit to intel-lab-lkp/linux that referenced this issue Apr 6, 2022
@samitolvanen @intel-lab-lkp
cfi: Use __builtin_function_start
ec50e03 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue Apr 7, 2022
@samitolvanen
cfi: Use __builtin_function_start
ea4411b 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
roxell pushed a commit to roxell/linux that referenced this issue Apr 14, 2022
@samitolvanen @kees
cfi: Use __builtin_function_start
e6f3b3c 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220405221618.633743-1-samitolvanen@google.com
samitolvanen added a commit to samitolvanen/linux that referenced this issue Apr 21, 2022
@samitolvanen
cfi: Use __builtin_function_start
230f81f 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue Apr 27, 2022
@samitolvanen
cfi: Use __builtin_function_start
ed8c02c 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue Apr 29, 2022
@samitolvanen
cfi: Use __builtin_function_start
8cf3fe7 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue May 3, 2022
@samitolvanen
cfi: Use __builtin_function_start
241ffb3 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue May 6, 2022
@samitolvanen
cfi: Use __builtin_function_start
6bbf423 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue May 9, 2022
@samitolvanen
cfi: Use __builtin_function_start
b9c8d82 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue May 10, 2022
@samitolvanen
cfi: Use __builtin_function_start
6c59033 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
samitolvanen added a commit to samitolvanen/linux that referenced this issue May 13, 2022
@samitolvanen
cfi: Use __builtin_function_start
fa90f7c 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
ptr1337 pushed a commit to CachyOS/linux that referenced this issue Jun 4, 2022
@samitolvanen @ptr1337
cfi: Use __builtin_function_start
f344430 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220405221618.633743-1-samitolvanen@google.com
ptr1337 pushed a commit to CachyOS/linux that referenced this issue Jun 9, 2022
@samitolvanen @ptr1337
cfi: Use __builtin_function_start
88e6fb8 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220405221618.633743-1-samitolvanen@google.com
github-actions bot pushed a commit to ararslan/llvm.vim that referenced this issue Sep 9, 2022
@samitolvanen
[llvm][IR] Add no_cfi constant
15a90c2 
With Control-Flow Integrity (CFI), the LowerTypeTests pass replaces
function references with CFI jump table references, which is a problem
for low-level code that needs the address of the actual function body.

For example, in the Linux kernel, the code that sets up interrupt
handlers needs to take the address of the interrupt handler function
instead of the CFI jump table, as the jump table may not even be mapped
into memory when an interrupt is triggered.

This change adds the no_cfi constant type, which wraps function
references in a value that LowerTypeTestsModule::replaceCfiUses does not
replace.

Link: ClangBuiltLinux/linux#1353

Reviewed By: nickdesaulniers, pcc

Differential Revision: https://reviews.llvm.org/D108478
xairy pushed a commit to xairy/linux that referenced this issue Dec 16, 2022
@samitolvanen
UPSTREAM: cfi: Use __builtin_function_start
c7eab89 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

Bug: 244543039
Bug: 244373730
Link: https://lore.kernel.org/r/20220405221618.633743-1-samitolvanen@google.com
(cherry picked from commit e6f3b3c)
Change-Id: I71d0e805fc66123107ff51f6e30d689948d64fa1
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
bzy-080408 pushed a commit to bzy-080408/android_pkvm_kernel that referenced this issue Jan 15, 2023
@samitolvanen
UPSTREAM: cfi: Use __builtin_function_start
63a2741 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux/linux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

Bug: 244543039
Bug: 244373730
Link: https://lore.kernel.org/r/20220405221618.633743-1-samitolvanen@google.com
(cherry picked from commit e6f3b3c)
Change-Id: I71d0e805fc66123107ff51f6e30d689948d64fa1
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
bzy-080408 pushed a commit to bzy-080408/android_pkvm_kernel that referenced this issue Jan 15, 2023
@samitolvanen @vdonnefort
FROMGIT: cfi: Use __builtin_function_start
08fe7e3 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux/linux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220405221618.633743-1-samitolvanen@google.com
bzy-080408 pushed a commit to bzy-080408/android_pkvm_kernel that referenced this issue Jan 15, 2023
@samitolvanen
UPSTREAM: cfi: Use __builtin_function_start
ae1b9a0 
Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: llvm/llvm-project@ec2e26e
Link: ClangBuiltLinux/linux#1353
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will@kernel.org> # arm64
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

Bug: 244543039
Bug: 244373730
Link: https://lore.kernel.org/r/20220405221618.633743-1-samitolvanen@google.com
(cherry picked from commit e6f3b3c)
Change-Id: I71d0e805fc66123107ff51f6e30d689948d64fa1
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
cuviper pushed a commit to cuviper/llvm.vim that referenced this issue Mar 1, 2023
@samitolvanen
(split) [llvm][IR] Add no_cfi constant
954312e 
With Control-Flow Integrity (CFI), the LowerTypeTests pass replaces
function references with CFI jump table references, which is a problem
for low-level code that needs the address of the actual function body.

For example, in the Linux kernel, the code that sets up interrupt
handlers needs to take the address of the interrupt handler function
instead of the CFI jump table, as the jump table may not even be mapped
into memory when an interrupt is triggered.

This change adds the no_cfi constant type, which wraps function
references in a value that LowerTypeTestsModule::replaceCfiUses does not
replace.

Link: ClangBuiltLinux/linux#1353

Reviewed By: nickdesaulniers, pcc

Differential Revision: https://reviews.llvm.org/D108478
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[FEATURE] CFI Related to building the kernel with Clang Control Flow Integrity feature-request Not a bug per-se [FIXED][LLVM] 14 This bug was fixed in LLVM 14.x
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@kees @samitolvanen @nickdesaulniers