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The recursive nature of __arm_lpae_{map,unmap}() means that
ARM_LPAE_BLOCK_SIZE() is evaluated for every level, including those
where block mappings aren't possible. This in itself is harmless enough,
as we will only ever be called with valid sizes from the pgsize_bitmap,
and thus always recurse down past any imaginary block sizes. The only
problem is that most of those imaginary sizes overflow the type used for
the calculation, and thus trigger warnings under UBsan:
[ 63.020939] ================================================================================
[ 63.021284] UBSAN: Undefined behaviour in drivers/iommu/io-pgtable-arm.c:312:22
[ 63.021602] shift exponent 39 is too large for 32-bit type 'int'
[ 63.021909] CPU: 0 PID: 1119 Comm: lkvm Not tainted 4.7.0-rc3+ #819
[ 63.022163] Hardware name: FVP Base (DT)
[ 63.022345] Call trace:
[ 63.022629] [<ffffff900808f258>] dump_backtrace+0x0/0x3a8
[ 63.022975] [<ffffff900808f614>] show_stack+0x14/0x20
[ 63.023294] [<ffffff90086bc9dc>] dump_stack+0x104/0x148
[ 63.023609] [<ffffff9008713ce8>] ubsan_epilogue+0x18/0x68
[ 63.023956] [<ffffff9008714410>] __ubsan_handle_shift_out_of_bounds+0x18c/0x1bc
[ 63.024365] [<ffffff900890fcb0>] __arm_lpae_map+0x720/0xae0
[ 63.024732] [<ffffff9008910170>] arm_lpae_map+0x100/0x190
[ 63.025049] [<ffffff90089183d8>] arm_smmu_map+0x78/0xc8
[ 63.025390] [<ffffff9008906c18>] iommu_map+0x130/0x230
[ 63.025763] [<ffffff9008bf7564>] vfio_iommu_type1_attach_group+0x4bc/0xa00
[ 63.026156] [<ffffff9008bf3c78>] vfio_fops_unl_ioctl+0x320/0x580
[ 63.026515] [<ffffff9008377420>] do_vfs_ioctl+0x140/0xd28
[ 63.026858] [<ffffff9008378094>] SyS_ioctl+0x8c/0xa0
[ 63.027179] [<ffffff9008086e70>] el0_svc_naked+0x24/0x28
[ 63.027412] ================================================================================
Perform the shift in a 64-bit type to prevent the theoretical overflow
and keep the peace. As it turns out, this generates identical code for
32-bit ARM, and marginally shorter AArch64 code, so it's good all round.
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.