Hi Miles, On 11/27/20 5:07 AM, Miles Chen wrote: > When we try to visit the pagemap of a tagged userspace pointer, we find > that the start_vaddr is not correct because of the tag. > To fix it, we should untag the usespace pointers in pagemap_read(). > Nit: s/usespace/userspace/ (please search and replace all occurrences :) ) > I tested with 5.10-rc4 and the issue remains. > > Explaination from Catalin in [1]: > Nit: s/Explaination/Explanation/ (please search and replace all occurrences :) ) > " > Arguably, that's a user-space bug since tagged file offsets were never > supported. In this case it's not even a tag at bit 56 as per the arm64 > tagged address ABI but rather down to bit 47. You could say that the > problem is caused by the C library (malloc()) or whoever created the > tagged vaddr and passed it to this function. It's not a kernel > regression as we've never supported it. > > Now, pagemap is a special case where the offset is usually not generated > as a classic file offset but rather derived by shifting a user virtual > address. I guess we can make a concession for pagemap (only) and allow > such offset with the tag at bit (56 - PAGE_SHIFT + 3). > " > > My test code is baed on [2]: Nit: s/baed/based/ (please search and replace all occurrences :) ) [...] > --- > fs/proc/task_mmu.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index 217aa2705d5d..92b277388f05 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf, > > src = *ppos; > svpfn = src / PM_ENTRY_BYTES; > - start_vaddr = svpfn << PAGE_SHIFT; > end_vaddr = mm->task_size; > > /* watch out for wraparound */ > - if (svpfn > mm->task_size >> PAGE_SHIFT) > + start_vaddr = end_vaddr; > + if (svpfn < (ULONG_MAX >> PAGE_SHIFT)) It seems that 'svpfn' should be less-then-equal (<=) '(ULONG_MAX >> PAGE_SHIFT)'. Is there any specific reason why this is not the case? > + start_vaddr = untagged_addr(svpfn << PAGE_SHIFT); > + > + /* Ensure the address is inside the task */ > + if (start_vaddr > mm->task_size) > start_vaddr = end_vaddr; > > /* > Otherwise: Reviewed-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx> -- Regards, Vincenzo
