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Partial writes to the NT_ARM_SVE and NT_ARM_SSVE regsets using an payload are handled inconsistently and non-deterministically. A comment within sve_set_common() indicates that we intended that a partial write would preserve any effective FPSIMD/SVE state which was not overwritten, but this has never worked consistently, and during syscalls the FPSIMD vector state may be non-deterministically preserved and may be erroneously migrated between streaming and non-streaming SVE modes. The simplest fix is to handle a partial write by consistently zeroing the remaining state. As detailed below I do not believe this will adversely affect any real usage. Neither GDB nor LLDB attempt partial writes to these regsets, and the documentation (in Documentation/arch/arm64/sve.rst) has always indicated that state preservation was not guaranteed, as is says: | The effect of writing a partial, incomplete payload is unspecified. When the logic was originally introduced in commit:43d4da2c45("arm64/sve: ptrace and ELF coredump support") ... there were two potential behaviours, depending on TIF_SVE: * When TIF_SVE was clear, all SVE state would be zeroed, excluding the low 128 bits of vectors shared with FPSIMD, FPSR, and FPCR. * When TIF_SVE was set, all SVE state would be zeroed, including the low 128 bits of vectors shared with FPSIMD, but excluding FPSR and FPCR. Note that as writing to NT_ARM_SVE would set TIF_SVE, partial writes to NT_ARM_SVE would not be idempotent, and if a first write preserved the low 128 bits, a subsequent (potentially identical) partial write would discard the low 128 bits. When support for the NT_ARM_SSVE regset was added in commit:e12310a0d3("arm64/sme: Implement ptrace support for streaming mode SVE registers") ... the above behaviour was retained for writes to the NT_ARM_SVE regset, though writes to the NT_ARM_SSVE would always zero the SVE registers and would not inherit FPSIMD register state. This happened as fpsimd_sync_to_sve() only copied the FPSIMD regs when TIF_SVE was clear and PSTATE.SM==0. Subsequently, when FPSIMD/SVE state tracking was changed across commits:baa8515281("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")a0136be443(arm64/fpsimd: Load FP state based on recorded data type")bbc6172eef("arm64/fpsimd: SME no longer requires SVE register state")8c845e2731("arm64/sve: Leave SVE enabled on syscall if we don't context switch") ... there was no corresponding update to the ptrace code, nor to fpsimd_sync_to_sve(), which stil considers TIF_SVE and PSTATE.SM rather than the saved fp_type. The saved state can be in the FPSIMD format regardless of whether TIF_SVE is set or clear, and the saved type can change non-deterministically during syscalls. Consequently a subsequent partial write to the NT_ARM_SVE or NT_ARM_SSVE regsets may non-deterministically preserve the FPSIMD state, and may migrate this state between streaming and non-streaming modes. Clean this up by never attempting to preserve ANY state when writing an SVE payload to the NT_ARM_SVE/NT_ARM_SSVE regsets, zeroing all relevant state including FPSR and FPCR. This simplifies the code, makes the behaviour deterministic, and avoids migrating state between streaming and non-streaming modes. As above, I do not believe this should adversely affect existing userspace applications. At the same time, remove fpsimd_sync_to_sve(). It is no longer used, doesn't do what its documentation implies, and gets in the way of other cleanups and fixes. Fixes:43d4da2c45("arm64/sve: ptrace and ELF coredump support") Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: David Spickett <david.spickett@arm.com> Cc: Luis Machado <luis.machado@arm.com> Cc: Marc Zyngier <maz@kernel.org> Cc: Mark Brown <broonie@kernel.org> Cc: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20250508132644.1395904-6-mark.rutland@arm.com Signed-off-by: Will Deacon <will@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.