diag(crash): name the library + offset a userspace SIGSEGV faults in

[Pryancito]

A dynamically-linked crash (labwc and its libraries) left only unhandled page fault @ … pc=0x7f… plus a Done(139) — the PC is an absolute address with no indication of which mapping it belongs to, so it could not be symbolised. Add VmAddressRegion::describe_addr, which locates the mapping containing an address and returns its base, the byte offset into the backing file (VMO), and the backing object's name (a file path for a file mapping, already set by the file get_vmo path; empty for anonymous memory).

The SIGSEGV handler now prints, for both the faulting PC and the bad data address:

[crash] pid=N pc 0x7f… in /usr/lib/libwlroots.so.13 + 0x1234 (map base 0x7f…)

so the exact fault can be located offline with addr2line -e <file> 0x<off> / objdump, turning an opaque compositor Done(139) into a pinned crash site. Anonymous mappings (heap/stack/JIT) print as [anon] + off.

Claude-Session: https://claude.ai/code/session_01MHCn5RNRcwR5PY1sYBHY2S

[Copilot]

Pull request overview

This PR improves userspace crash diagnostics by resolving faulting addresses (PC and fault address) to the owning VM mapping, including mapping base, offset into the backing VMO/file, and the backing object name, so dynamically-linked crashes can be symbolized offline.

Changes:

• Add VmAddressRegion::describe_addr to locate the mapping containing a virtual address and return (base, offset, name).
• Extend the userspace SIGSEGV/page-fault path to log the resolved library/anon mapping and offset for both pc and the fault address.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
zircon-object/src/vm/vmar.rs	Adds describe_addr helper to map an address to its mapping base, VMO/file offset, and object name.
loader/src/linux.rs	Logs resolved mapping information for crash PC and fault address to enable offline symbolization.

Suppressed comments (1)

zircon-object/src/vm/vmar.rs:1531

• file_off is computed with saturating_sub, which can silently produce an incorrect offset (e.g., if invariants are ever violated and vaddr < inner.addr, this would report the mapping start instead of failing). Since this method already returns Option, prefer checked_sub/checked_add and return None on underflow/overflow to avoid misleading crash diagnostics.

        // File offset of `vaddr`: the VMO offset the mapping starts at plus how
        // far `vaddr` is into the mapping.
        let file_off = inner.vmo_offset + vaddr.saturating_sub(inner.addr);
        Some((inner.addr, file_off, map.vmo.name()))

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