Changes in binfmt_misc_loader.c

b9cba7ebfe539f3e4bbdd03a1e0efa3b30b3f592 - Merge tag 'vfs-7.3-rc1.binfmt' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Mon, 17 Aug 2026 17:35:25 +0200

Merge tag 'vfs-7.3-rc1.binfmt' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfsPull binfmt updates from Christian Brauner: "This contains a bunch of work for binfmt_misc. It fixes a bunch of old bugs, reworks the locking, and then extends the format registry so a binary type can be matched programmatically and its interpreter computed per exec instead of being a fixed string recorded at registration time. This allows nixos and other to e.g., implement relocatable binaries meaning the interpreter/dynamic loader can be determined programatically, say found relative to the binary. The mechanism is flexible and can support other policies: - Handler lookup is now an rcu walk. An exec that matches no binfmt_misc entry should now never write to a shared cacheline - remove the VERBOSE_STATUS and USE_DEBUG compile time toggles - convert the entry file to a seq_file which simplifies things quite a bit and kills a lot of custom logic - make flags proper enums - rename struct Node to binfmt_misc_entry - allow entries to be removed with unlink(2) - Add the ability to attach bpf programs to binfmt_misc entries so it's possible to dynamically choose the execution environment such as the loader or interpreter on a per binary basis. A handler is an instance of a binfmt_misc_ops struct_ops with a ->match() and a ->load() program. match() decides from the entry lookup walk whether the handler applies under the same registration-order. It can read file content as needed not only the prefetched 256 bytes in bprm->buf. load() then selects the interpreter and stages it through the new bpf_binprm_set_interp(), bpf_binprm_set_interp_arg() and bpf_binprm_set_flags() kfuncs. Handlers are published in a registry keyed by the registering task's user namespace and activated through the existing text interface with a new 'B' type carrying the handler name: echo ':origin:B::::nix:' > /proc/sys/fs/binfmt_misc/register The permission and namespacing model is unchanged. Activating a handler requires the same write access to an instance as any other registration. A container mounting its own instance escapes the host's entries exactly as before. The computed interpreter is opened with open_exec() under the caller's credentials and goes through full LSM vetting as the next binprm level. A program can only ever redirect the caller to something the caller could exec anyway. - Two dispatch modes are added. So far the chosen interpreter owns the whole process identity (argv[0], /proc/pid/cmdline, /proc/self/exe all name interpreter information). So relocatable find the dynamic linker instead. Also a binary passed to execveat() as an inaccessible O_CLOEXEC fd cannot run at all and gdb trips because AT_ENTRY and AT_PHDR do not match the exe file. So PIE symbols are unrelocated. This adds transparent dispatch which allows the interpreter to load the binary through AT_EXECFD and leaves the argument vector exactly as the caller built it and labels mm->exe_file and comm with the binary. It also raises the AT_FLAGS_TRANSPARENT_INTERP aux vector bit. The interpreter keeps control of mapping the binary. The second mode is loader substitution. This allows a binary to be executed natively and only the interpreter to be changed. - Last, interpreters can be bound at registration time. Each interpreter is opened by its own write with the credentials the entry file was opened with. The program picks one per exec with bpf_binprm_select_interp(). Ucounts are used to properly account for pre-opened interpreters via /proc/sys/user/max_binfmt_misc_interpreters"* tag 'vfs-7.3-rc1.binfmt' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (63 commits) binfmt_misc: document the pre-opened interpreter limit selftests/exec: test the pre-opened interpreter limit binfmt_misc: correctly account pre-opened interpreters binfmt_misc: document interpreters bound by a 'B' entry selftests/exec: test interpreters bound to a 'B' entry binfmt_misc: let a 'B' entry bind its interpreters binfmt_misc: carry pre-opened interpreters in struct binfmt_misc_interp selftests/exec: share the bpf handler preconditions binfmt_misc: document registering an entry disabled selftests/exec: test registering an entry disabled selftests/exec: let binfmt_flag_supported() return a bool selftests/exec: check that a binfmt_misc instance cannot be pinned binfmt_misc: let a register string create an entry disabled binfmt_misc: document loader substitution selftests/exec: test binfmt_misc loader substitution binfmt_misc: let a bpf handler request loader substitution binfmt_misc: add the 'L' loader substitution flag binfmt_elf_fdpic: consume a stashed PT_INTERP substitute binfmt_elf: consume a stashed PT_INTERP substitute exec: carry a PT_INTERP substitute in struct linux_binprm ... List of files: /linux/tools/testing/selftests/exec/binfmt_misc_loader.c

b2a52381541af862076b5e7d17db2ca76e921e95 - Merge patch series "binfmt_misc: transparent interpreters and PT_INTERP loader substitution"

Sat, 25 Jul 2026 11:11:48 +0200

Merge patch series "binfmt_misc: transparent interpreters and PT_INTERP loader substitution"Christian Brauner says:binfmt_misc has exactly one execution model where the registeredinterpreter becomes the executed program and the matched binary ishanded to it as an argument. For wine or qemu-user that is the point.For a per-binary loader it is backwards. The interpreter is animplementation detail of running the binary, yet it owns the entireprocess identity:- argv[0] and /proc/pid/cmdline show the interpreter invocation, not what the caller executed.- /proc/self/exe names the interpreter. Relocatable programs commonly locate themselves through it and find the dynamic linker instead.- A binary passed to execveat() as an inaccessible O_CLOEXEC fd cannot run at all as the interpreter has no path to open it by.- gdb cross-validates AT_ENTRY/AT_PHDR against the exe file and discards the load displacement on mismatch leaving PIE symbols unrelocated.This series adds two dispatch modes that close the gap from oppositeends:(1) transparent dispatch Registered with the 'T' flag or chosen per exec with BPF_BINPRM_TRANSPARENT. The binary is sent to the interpreter through AT_EXECFD, the argument vector stays exactly as the caller built it, and the kernel labels mm->exe_file and comm with the binary. A new AT_FLAGS_TRANSPARENT_INTERP aux vector bit is raised indicating that nothing was spliced, argv belongs to the program, and to load it from the descriptor. The interpreter keeps control of mapping the binary, so the mode covers foreign architectures and non-ELF payloads. The exe label is not a new privilege. It names precisely the file the caller passed to execve(), not a file of the process's choosing. That file is permission-checked, write-denied while the process runs and recorded by audit. Credential derivation does not change exactly as today.(2) loader substitution The kernel executes the matched binary natively as the main image and substitutes the registered interpreter for the binary's PT_INTERP. binfmt_misc functions as a PT_INTERP override. There is no contract and no identity to reconstruct. So a stock dynamic loader works unchanged. Hence, 'L' is for native-arch ELF with PT_INTERP.The two modes compose. A bpf handler reads the ELF header from bprm->bufand grades per binary, picking 'L' where it applies and 'T' or classicdispatch for the rest. If userspace control over relocation is wanted'T' is the way to go.* patches from https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-0-e57866e4ae0f@kernel.org: (21 commits) binfmt_misc: document loader substitution selftests/exec: test binfmt_misc loader substitution binfmt_misc: let a bpf handler request loader substitution binfmt_misc: add the 'L' loader substitution flag binfmt_elf_fdpic: consume a stashed PT_INTERP substitute binfmt_elf: consume a stashed PT_INTERP substitute exec: carry a PT_INTERP substitute in struct linux_binprm binfmt_misc: document the transparent identity contract selftests/exec: test the transparent binfmt_misc mode binfmt_misc: let a bpf handler run the interpreter transparently binfmt_misc: add a static transparent flag 'T' binfmt_misc: add transparent interpreter dispatch exec: label mm->exe_file with the binary for a transparent dispatch exec: add AT_FLAGS_TRANSPARENT_INTERP selftests/exec: convert the binfmt_misc bpf test to the kselftest harness exec: release the replaced file with do_close_execat() binfmt_misc: split out entry_open_interpreter() and build_interp_argv() binfmt_misc: normalize the per-exec invocation flags binfmt_misc: table-drive the register string flags docs, binfmt_misc: keep general usage out of the handler sections ...Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-0-e57866e4ae0f@kernel.orgSigned-off-by: Christian Brauner (Amutable) List of files: /linux/tools/testing/selftests/exec/binfmt_misc_loader.c

87c50a5855cf0e1a4a42448b2245f6e90df20a4c - selftests/exec: test binfmt_misc loader substitution

Tue, 21 Jul 2026 16:14:02 +0200

selftests/exec: test binfmt_misc loader substitutionExercise the 'L' flag end to end. The payload runs as the main imagewith a copy of the system loader substituted for its PT_INTERP, andasserts the native identity from inside:- argv exactly as the caller built it- no AT_EXECFD- AT_FLAGS clear- AT_BASE set but outside its own image- AT_PHDR/AT_ENTRY inside it- /proc/self/{exe,comm,stat} and AT_EXECFN all describing the binary- ETXTBSY on the running binary- the substituted loader visible in /proc/self/maps under its real pathMagic matching pokes a marker into the ELF header's e_ident padding(EI_PAD, offset 9), which sits inside the match window and is ignored bykernel and loader alike. the same binary is also matched by extension.Two cases cover the paths where the substitution does not happen. A '#!'file that matched an 'L' entry is claimed by binfmt_script rather than bybinfmt_elf, so the staged substitute has to be released when theinterpreter replaces the file; the test opens the loader for writingafterwards, which fails with ETXTBSY if the write denial was leakedinstead. A relative interpreter path is rejected at registration for both'L' and 'C', neither of which may resolve one against the workingdirectory of whoever runs the binary.The bpf-side BPF_BINPRM_LOADER path shares all machinery past the flagmapping. A harness case for it can join the bpf runtime coverage ofthe transparent series.Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-20-e57866e4ae0f@kernel.orgSigned-off-by: Christian Brauner (Amutable) List of files: /linux/tools/testing/selftests/exec/binfmt_misc_loader.c