Re: [PATCH bpf-next v2 2/2] bpftool: Make the LLVM disassembler an optional runtime dependency

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] bpftool: llvm_plugin_find() misses staged plugins and prefers system plugin
--

commit aa662f293876360fc9cf9139e86022f88a5bbb64
Author: Mark Korenberg <socketpair@gmail.com>

bpftool: Make the LLVM disassembler an optional runtime dependency

This commit moves the libLLVM-based JIT disassembler into an optional
shared library plugin (bpftool-llvm.so) loaded on demand via dlopen()
when disassembling JITed programs or checking feature availability.

Link: https://github.com/libbpf/bpftool/issues/262

> diff --git a/tools/bpf/bpftool/jit_disasm.c b/tools/bpf/bpftool/jit_disasm.c
> index 5ff144ce84a1c..2691252a115b9 100644
> --- a/tools/bpf/bpftool/jit_disasm.c
> +++ b/tools/bpf/bpftool/jit_disasm.c

[ ... ]

> @@ -45,7 +45,105 @@ static int oper_count;

[ ... ]

> +/*
> + * Look for the plugin in its install directory, then next to the bpftool
> + * binary so that bpftool also works from its build directory. Do not look
> + * in the current directory: bpftool usually runs as root, and would then
> + * load any library planted in the directory it is run from.
> + */
> +static int llvm_plugin_find(char *path, size_t size)
> +{
> +	char exe[PATH_MAX];
> +	ssize_t len;
> +	char *dir;
> +	int n;
> +
> +	snprintf(path, size, "%s/%s", LLVM_PLUGIN_DIR, LLVM_PLUGIN_NAME);
> +	if (!access(path, F_OK))
> +		return 0;
> +
> +	len = readlink("/proc/self/exe", exe, sizeof(exe) - 1);
> +	if (len < 0)
> +		return -1;
> +	exe[len] = '\0';
> +	dir = strrchr(exe, '/');
> +	if (!dir)
> +		return -1;
> +	*dir = '\0';
> +
> +	n = snprintf(path, size, "%s/%s", exe, LLVM_PLUGIN_NAME);
> +	if (n < 0 || (size_t)n >= size)
> +		return -1;
> +	return access(path, F_OK);
> +}

[Severity: Medium]
Can llvm_plugin_find() fail to locate bpftool-llvm.so when bpftool is
installed into a staging directory via DESTDIR, such as in the BPF
selftests build?

In tools/testing/selftests/bpf/Makefile, $(DEFAULT_BPFTOOL) and
$(CROSS_BPFTOOL) are built and staged with:

tools/testing/selftests/bpf/Makefile:
	$(Q)$(MAKE) $(submake_extras)  -C $(BPFTOOLDIR)			       \
		    ...
		    OUTPUT=$(HOST_BUILD_DIR)/bpftool/			       \
		    ...
		    prefix= DESTDIR=$(HOST_SCRATCH_DIR)/ install-bin

Because prefix= is empty and libdir is not overridden, tools/bpf/bpftool/Makefile
sets libdir to /lib and compiles jit_disasm.o with:

tools/bpf/bpftool/Makefile:
	prefix ?= /usr/local
	libdir ?= $(prefix)/lib
	...
	CFLAGS += -DLLVM_PLUGIN_DIR='"$(libdir)/bpftool"'

Meanwhile, the install-bin target places the binary and plugin in separate
directories under $(DESTDIR):

tools/bpf/bpftool/Makefile:
	install-bin: $(OUTPUT)bpftool $(LLVM_PLUGIN)
		$(call QUIET_INSTALL, bpftool)
		$(Q)$(INSTALL) -m 0755 -d $(DESTDIR)$(prefix)/sbin
		$(Q)$(INSTALL) $(OUTPUT)bpftool $(DESTDIR)$(prefix)/sbin/bpftool
	ifneq ($(LLVM_PLUGIN),)
		$(call QUIET_INSTALL, bpftool-llvm.so)
		$(Q)$(INSTALL) -m 0755 -D -t $(DESTDIR)$(libdir)/bpftool $(LLVM_PLUGIN)
	endif

This installs bpftool to $(HOST_SCRATCH_DIR)/sbin/bpftool and the plugin to
$(HOST_SCRATCH_DIR)/lib/bpftool/bpftool-llvm.so. When the staged bpftool
runs, llvm_plugin_find() checks /lib/bpftool/bpftool-llvm.so and
$(HOST_SCRATCH_DIR)/sbin/bpftool-llvm.so, missing the staged plugin in
$(HOST_SCRATCH_DIR)/lib/bpftool/bpftool-llvm.so (causing "bpftool prog dump
jited" to fail and "bpftool version" to omit the llvm feature).

Additionally, because llvm_plugin_find() checks LLVM_PLUGIN_DIR before the
directory of /proc/self/exe, wouldn't running an uninstalled ./bpftool from
its build directory on a host where $(LLVM_PLUGIN_DIR)/bpftool-llvm.so is
already installed load the system plugin instead of ./bpftool-llvm.so next
to the binary (and fail without falling back if the installed plugin has a
different BPFTOOL_LLVM_ABI_VERSION or incompatible sanitizer flags)?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261029-bpftool-plugin-v2-0-1a47b2391b0b@gmail.com?part=2