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5665bc35c1
The sc and scv 0 system calls have different ABI conventions, and
ptracers need to know which system call type is being used if they want
to look at the syscall registers.
Document that pt_regs.trap can be used for this, and fix one in-tree user
to work with scv 0 syscalls.
Fixes: 7fa95f9ada ("powerpc/64s: system call support for scv/rfscv instructions")
Cc: stable@vger.kernel.org # v5.9+
Reported-by: "Dmitry V. Levin" <ldv@altlinux.org>
Suggested-by: "Dmitry V. Levin" <ldv@altlinux.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210520111931.2597127-1-npiggin@gmail.com
153 lines
6.4 KiB
ReStructuredText
153 lines
6.4 KiB
ReStructuredText
===============================================
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Power Architecture 64-bit Linux system call ABI
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===============================================
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syscall
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=======
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Invocation
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----------
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The syscall is made with the sc instruction, and returns with execution
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continuing at the instruction following the sc instruction.
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If PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector, the
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scv 0 instruction is an alternative that may provide better performance,
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with some differences to calling sequence.
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syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI
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specification C function calling sequence, including register preservation
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rules, with the following differences.
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.. [1] Some syscalls (typically low-level management functions) may have
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different calling sequences (e.g., rt_sigreturn).
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Parameters
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----------
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The system call number is specified in r0.
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There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
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Return value
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------------
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- For the sc instruction, both a value and an error condition are returned.
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cr0.SO is the error condition, and r3 is the return value. When cr0.SO is
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clear, the syscall succeeded and r3 is the return value. When cr0.SO is set,
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the syscall failed and r3 is the error value (that normally corresponds to
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errno).
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- For the scv 0 instruction, the return value indicates failure if it is
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-4095..-1 (i.e., it is >= -MAX_ERRNO (-4095) as an unsigned comparison),
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in which case the error value is the negated return value.
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Stack
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-----
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System calls do not modify the caller's stack frame. For example, the caller's
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stack frame LR and CR save fields are not used.
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Register preservation rules
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---------------------------
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Register preservation rules match the ELF ABI calling sequence with some
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differences.
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For the sc instruction, the differences from the ELF ABI are as follows:
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+--------------+--------------------+-----------------------------------------+
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| Register | Preservation Rules | Purpose |
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+==============+====================+=========================================+
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| r0 | Volatile | (System call number.) |
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+--------------+--------------------+-----------------------------------------+
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| r3 | Volatile | (Parameter 1, and return value.) |
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+--------------+--------------------+-----------------------------------------+
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| r4-r8 | Volatile | (Parameters 2-6.) |
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+--------------+--------------------+-----------------------------------------+
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| cr0 | Volatile | (cr0.SO is the return error condition.) |
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+--------------+--------------------+-----------------------------------------+
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| cr1, cr5-7 | Nonvolatile | |
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+--------------+--------------------+-----------------------------------------+
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| lr | Nonvolatile | |
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+--------------+--------------------+-----------------------------------------+
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For the scv 0 instruction, the differences from the ELF ABI are as follows:
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+--------------+--------------------+-----------------------------------------+
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| Register | Preservation Rules | Purpose |
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+==============+====================+=========================================+
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| r0 | Volatile | (System call number.) |
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+--------------+--------------------+-----------------------------------------+
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| r3 | Volatile | (Parameter 1, and return value.) |
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+--------------+--------------------+-----------------------------------------+
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| r4-r8 | Volatile | (Parameters 2-6.) |
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+--------------+--------------------+-----------------------------------------+
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All floating point and vector data registers as well as control and status
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registers are nonvolatile.
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Transactional Memory
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--------------------
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Syscall behavior can change if the processor is in transactional or suspended
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transaction state, and the syscall can affect the behavior of the transaction.
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If the processor is in suspended state when a syscall is made, the syscall
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will be performed as normal, and will return as normal. The syscall will be
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performed in suspended state, so its side effects will be persistent according
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to the usual transactional memory semantics. A syscall may or may not result
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in the transaction being doomed by hardware.
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If the processor is in transactional state when a syscall is made, then the
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behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
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auxiliary vector.
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- If present, which is the case for newer kernels, then the syscall will not
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be performed and the transaction will be doomed by the kernel with the
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failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR.
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- If not present (older kernels), then the kernel will suspend the
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transactional state and the syscall will proceed as in the case of a
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suspended state syscall, and will resume the transactional state before
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returning to the caller. This case is not well defined or supported, so this
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behavior should not be relied upon.
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scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC.
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ptrace
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------
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When ptracing system calls (PTRACE_SYSCALL), the pt_regs.trap value contains
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the system call type that can be used to distinguish between sc and scv 0
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system calls, and the different register conventions can be accounted for.
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If the value of (pt_regs.trap & 0xfff0) is 0xc00 then the system call was
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performed with the sc instruction, if it is 0x3000 then the system call was
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performed with the scv 0 instruction.
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vsyscall
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========
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vsyscall calling sequence matches the syscall calling sequence, with the
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following differences. Some vsyscalls may have different calling sequences.
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Parameters and return value
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---------------------------
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r0 is not used as an input. The vsyscall is selected by its address.
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Stack
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-----
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The vsyscall may or may not use the caller's stack frame save areas.
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Register preservation rules
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---------------------------
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=========== ========
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r0 Volatile
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cr1, cr5-7 Volatile
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lr Volatile
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=========== ========
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Invocation
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----------
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The vsyscall is performed with a branch-with-link instruction to the vsyscall
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function address.
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Transactional Memory
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--------------------
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vsyscalls will run in the same transactional state as the caller. A vsyscall
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may or may not result in the transaction being doomed by hardware.
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