kl-addr

Analyse an address

Canonical: kernlens-addr
Category: addresses & page tables
Synopsis: kl-addr [-h] [-b] address [tag_mask]

Every question KernLens can answer about a bare address: canonicality, the region it falls in, whether it is an ERR_PTR, how it aligns, its pointer tag, and -- for a direct-map address -- the physical address, pfn, and struct page. --brief stops after the region and the translation, which is what kl-page printed.

Options

Option	Argument	Meaning
	—	address or GDB expression to analyse
	(optional)	mask used for pointer-tag analysis (e.g. 0xfff)
-b, --brief	—	only the class, the translation, and the two bases

The full analysis

Invocation

kl-addr 0xffff90c340400000

Output

Address: 0xffff90c340400000
Canonical: 48=True  57=True
Region: kernel=True user=False direct=True vmemmap=False vmalloc=False module=False text=False
ERR_PTR: no
Alignment: down=0xffff90c340400000 up=0xffff90c340400000
Physical: 0x400000  pfn=1024
page: 0xffffcd3b80010000

Reading it

Canonical checks both widths, because a 57-bit kernel accepts addresses a 48-bit one would reject as non-canonical. Region is the set of range tests that apply, and direct=True is what makes the translation below meaningful. Alignment gives the containing page and the next boundary.

Pointer tags

Invocation

kl-addr 0xffff90c340400000 0xfff

Output

Address: 0xffff90c340400000
Canonical: 48=True  57=True
Region: kernel=True user=False direct=True vmemmap=False vmalloc=False module=False text=False
ERR_PTR: no
Alignment: down=0xffff90c340400000 up=0xffff90c340400000
Tags: mask=0xfff tag=0x0 stripped=0xffff90c340400000
Physical: 0x400000  pfn=1024
page: 0xffffcd3b80010000

Reading it

The optional mask separates a tagged pointer from the address inside it -- the last step, for a value that carried a page offset or a tag, back to the page it names.

The brief form

Invocation

kl-addr 0xffff90c340400000 --brief

Output

Address: 0xffff90c340400000
Class: kernel=True user=False direct=True vmemmap=False vmalloc=False module=False text=False
Physical: 0x400000  pfn=1024
page: 0xffffcd3b80010000
bases: page_offset=0xffff90c340000000 vmemmap=0xffffcd3b80000000

Reading it

Class is the region set, Physical and page follow from the direct map and the vmemmap, and bases: prints page_offset_base and vmemmap_base so the arithmetic can be checked by hand: page = vmemmap + pfn * sizeof(struct page). sizeof(struct page) is read from the debug info (64 bytes here), never assumed: it changes with CONFIG_* options, and using PAGE_SIZE instead is a classic error that produces struct page pointers 64x too far apart.

Notes

The region boundaries are constants (KERNEL_BASE, VMALLOC_START, ...) for the layout's fixed parts, while page_offset_base and vmemmap_base are read from the kernel. A wrong slide changes the former, never the latter.

---

Part of the KernLens tutorial. Every output block on this page was captured from the kernel described in the tutorial index.