From: Maciej Wieczor-Retman <[email protected]> With the announcement of ChkTag, it's worth preparing a stable x86 linear address masking (lam) user interface. One important aspect of lam is the tag width, and aligning it with other industry solutions can provide a more popular, generalized interface that other technologies could utilize.
ChkTag will use 4-bit tags and since that's the direction other memory tagging implementations seem to be taking too (for example Arm's MTE) it's reasonable to converge lam in linux to the same specification. Even though x86's LAM supports 6-bit tags it is beneficial to shorten lam to 4 bits as ChkTag will likely be the main user of the interface and such connection should simplify things in the future. Shrink the maximum acceptable tag width from 6 to 4. Signed-off-by: Maciej Wieczor-Retman <[email protected]> --- Changelog v4: - Ditch the default wording in the patch message. - Add the imperative last line as Dave suggested. Changelog v3: - Remove the variability of the lam width after the debugfs part was removed from the patchset. arch/x86/kernel/process_64.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 08e72f429870..1a0e96835bbc 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -797,7 +797,7 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr) #ifdef CONFIG_ADDRESS_MASKING -#define LAM_U57_BITS 6 +#define LAM_DEFAULT_BITS 4 static void enable_lam_func(void *__mm) { @@ -814,7 +814,7 @@ static void enable_lam_func(void *__mm) static void mm_enable_lam(struct mm_struct *mm) { mm->context.lam_cr3_mask = X86_CR3_LAM_U57; - mm->context.untag_mask = ~GENMASK(62, 57); + mm->context.untag_mask = ~GENMASK(57 + LAM_DEFAULT_BITS - 1, 57); /* * Even though the process must still be single-threaded at this @@ -850,7 +850,7 @@ static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits) return -EBUSY; } - if (!nr_bits || nr_bits > LAM_U57_BITS) { + if (!nr_bits || nr_bits > LAM_DEFAULT_BITS) { mmap_write_unlock(mm); return -EINVAL; } @@ -965,7 +965,7 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2) if (!cpu_feature_enabled(X86_FEATURE_LAM)) return put_user(0, (unsigned long __user *)arg2); else - return put_user(LAM_U57_BITS, (unsigned long __user *)arg2); + return put_user(LAM_DEFAULT_BITS, (unsigned long __user *)arg2); #endif case ARCH_SHSTK_ENABLE: case ARCH_SHSTK_DISABLE: -- 2.53.0
