kprobe attach is name-based, using lookups of kallsyms to translate
a function name to an address. Currently uprobe attach is done
via an offset value as described in [1]. Extend uprobe opts
for attach to include a function name which can then be converted
into a uprobe-friendly offset. The calcualation is done in two
steps:
- first, determine the symbol address using libelf; this gives us
the offset as reported by objdump; then, in the case of local
functions
- subtract the base address associated with the object, retrieved
from ELF program headers.
The resultant value is then added to the func_offset value passed
in to specify the uprobe attach address. So specifying a func_offset
of 0 along with a function name "printf" will attach to printf entry.
The modes of operation supported are to attach to a local function
in a binary - function "foo1" in /usr/bin/foo - or to attach to
a library function in a shared object - function "malloc" in
/usr/lib64/libc.so.6. Because the symbol table values of shared
object functions in a binary will be 0, we cannot attach to a
shared object function in a binary ("malloc" in /usr/bin/foo).
[1] https://www.kernel.org/doc/html/latest/trace/uprobetracer.html
Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
tools/lib/bpf/libbpf.c | 199 +++++++++++++++++++++++++++++++++++++++++++++++++
tools/lib/bpf/libbpf.h | 10 ++-
2 files changed, 208 insertions(+), 1 deletion(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index fdb3536..6479aae 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -10183,6 +10183,191 @@ static int perf_event_uprobe_open_legacy(const char *probe_name, bool retprobe,
return pfd;
}
+/* uprobes deal in relative offsets; subtract the base address associated with
+ * the mapped binary. See Documentation/trace/uprobetracer.rst for more
+ * details.
+ */
+static ssize_t find_elf_relative_offset(Elf *elf, ssize_t addr)
+{
+ size_t n;
+ int i;
+
+ if (elf_getphdrnum(elf, &n)) {
+ pr_warn("elf: failed to find program headers: %s\n",
+ elf_errmsg(-1));
+ return -ENOENT;
+ }
+
+ for (i = 0; i < n; i++) {
+ int seg_start, seg_end, seg_offset;
+ GElf_Phdr phdr;
+
+ if (!gelf_getphdr(elf, i, &phdr)) {
+ pr_warn("elf: failed to get program header %d: %s\n",
+ i, elf_errmsg(-1));
+ return -ENOENT;
+ }
+ if (phdr.p_type != PT_LOAD || !(phdr.p_flags & PF_X))
+ continue;
+
+ seg_start = phdr.p_vaddr;
+ seg_end = seg_start + phdr.p_memsz;
+ seg_offset = phdr.p_offset;
+ if (addr >= seg_start && addr < seg_end)
+ return (ssize_t)addr - seg_start + seg_offset;
+ }
+ pr_warn("elf: failed to find prog header containing 0x%lx\n", addr);
+ return -ENOENT;
+}
+
+/* Return next ELF section of sh_type after scn, or first of that type
+ * if scn is NULL.
+ */
+static Elf_Scn *find_elfscn(Elf *elf, int sh_type, Elf_Scn *scn)
+{
+ Elf64_Shdr *sh;
+
+ while ((scn = elf_nextscn(elf, scn)) != NULL) {
+ sh = elf64_getshdr(scn);
+ if (sh && sh->sh_type == sh_type)
+ break;
+ }
+ return scn;
+}
+
+/* Find offset of function name in object specified by path. "name" matches
+ * symbol name or name@@LIB for library functions.
+ */
+static ssize_t find_elf_func_offset(const char *binary_path, const char *name)
+{
+ size_t si, strtabidx, nr_syms;
+ bool dynamic, is_shared_lib;
+ char errmsg[STRERR_BUFSIZE];
+ Elf_Data *symbols = NULL;
+ int lastbind = -1, fd;
+ ssize_t ret = -ENOENT;
+ Elf_Scn *scn = NULL;
+ const char *sname;
+ Elf64_Shdr *sh;
+ GElf_Ehdr ehdr;
+ Elf *elf;
+
+ if (!binary_path) {
+ pr_warn("name-based attach requires binary_path\n");
+ return -EINVAL;
+ }
+ if (elf_version(EV_CURRENT) == EV_NONE) {
+ pr_warn("elf: failed to init libelf for %s\n", binary_path);
+ return -LIBBPF_ERRNO__LIBELF;
+ }
+ fd = open(binary_path, O_RDONLY | O_CLOEXEC);
+ if (fd < 0) {
+ ret = -errno;
+ pr_warn("failed to open %s: %s\n", binary_path,
+ libbpf_strerror_r(ret, errmsg, sizeof(errmsg)));
+ return ret;
+ }
+ elf = elf_begin(fd, ELF_C_READ_MMAP, NULL);
+ if (!elf) {
+ pr_warn("elf: could not read elf from %s: %s\n",
+ binary_path, elf_errmsg(-1));
+ close(fd);
+ return -LIBBPF_ERRNO__FORMAT;
+ }
+ if (!gelf_getehdr(elf, &ehdr)) {
+ pr_warn("elf: failed to get ehdr from %s: %s\n",
+ binary_path, elf_errmsg(-1));
+ ret = -LIBBPF_ERRNO__FORMAT;
+ goto out;
+ }
+ is_shared_lib = ehdr.e_type == ET_DYN;
+ dynamic = is_shared_lib;
+retry:
+ scn = find_elfscn(elf, dynamic ? SHT_DYNSYM : SHT_SYMTAB, NULL);
+ if (!scn) {
+ pr_warn("elf: failed to find symbol table ELF section in %s\n",
+ binary_path);
+ ret = -ENOENT;
+ goto out;
+ }
+
+ sh = elf64_getshdr(scn);
+ strtabidx = sh->sh_link;
+ symbols = elf_getdata(scn, 0);
+ if (!symbols) {
+ pr_warn("elf: failed to get symtab section in %s: %s\n",
+ binary_path, elf_errmsg(-1));
+ ret = -LIBBPF_ERRNO__FORMAT;
+ goto out;
+ }
+
+ lastbind = -1;
+ nr_syms = symbols->d_size / sizeof(Elf64_Sym);
+ for (si = 0; si < nr_syms; si++) {
+ Elf64_Sym *sym = (Elf64_Sym *)symbols->d_buf + si;
+ size_t matchlen;
+ int currbind;
+
+ if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
+ continue;
+
+ sname = elf_strptr(elf, strtabidx, sym->st_name);
+ if (!sname) {
+ pr_warn("elf: failed to get sym name string in %s\n",
+ binary_path);
+ ret = -EIO;
+ goto out;
+ }
+ currbind = ELF64_ST_BIND(sym->st_info);
+
+ /* If matching on func@@LIB, match on everything prior to
+ * the '@@'; otherwise match on full string.
+ */
+ matchlen = strstr(sname, "@@") ? strstr(sname, "@@") - sname :
+ strlen(sname);
+
+ if (strlen(name) == matchlen &&
+ strncmp(sname, name, matchlen) == 0) {
+ if (ret >= 0 && lastbind != -1) {
+ /* handle multiple matches */
+ if (lastbind != STB_WEAK && currbind != STB_WEAK) {
+ /* Only accept one non-weak bind. */
+ pr_warn("elf: additional match for '%s': %s\n",
+ sname, name);
+ ret = -LIBBPF_ERRNO__FORMAT;
+ goto out;
+ } else if (currbind == STB_WEAK) {
+ /* already have a non-weak bind, and
+ * this is a weak bind, so ignore.
+ */
+ continue;
+ }
+ }
+ ret = sym->st_value;
+ lastbind = currbind;
+ }
+ }
+ if (ret == 0) {
+ if (!dynamic) {
+ dynamic = true;
+ goto retry;
+ }
+ pr_warn("elf: '%s' is 0 in symbol table; try using shared library path instead of '%s'\n",
+ name, binary_path);
+ ret = -ENOENT;
+ }
+ if (ret > 0) {
+ pr_debug("elf: symbol table match for '%s': 0x%lx\n",
+ name, ret);
+ if (!is_shared_lib)
+ ret = find_elf_relative_offset(elf, ret);
+ }
+out:
+ elf_end(elf);
+ close(fd);
+ return ret;
+}
+
LIBBPF_API struct bpf_link *
bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
const char *binary_path, size_t func_offset,
@@ -10194,6 +10379,7 @@ static int perf_event_uprobe_open_legacy(const char *probe_name, bool retprobe,
size_t ref_ctr_off;
int pfd, err;
bool retprobe, legacy;
+ const char *func_name;
if (!OPTS_VALID(opts, bpf_uprobe_opts))
return libbpf_err_ptr(-EINVAL);
@@ -10202,6 +10388,19 @@ static int perf_event_uprobe_open_legacy(const char *probe_name, bool retprobe,
ref_ctr_off = OPTS_GET(opts, ref_ctr_offset, 0);
pe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
+ func_name = OPTS_GET(opts, func_name, NULL);
+ if (func_name) {
+ ssize_t sym_off;
+
+ sym_off = find_elf_func_offset(binary_path, func_name);
+ if (sym_off < 0) {
+ pr_debug("could not find sym offset for %s in %s\n",
+ func_name, binary_path);
+ return libbpf_err_ptr(sym_off);
+ }
+ func_offset += (size_t)sym_off;
+ }
+
legacy = determine_uprobe_perf_type() < 0;
if (!legacy) {
pfd = perf_event_open_probe(true /* uprobe */, retprobe, binary_path,
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 9728551..4675586 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -431,9 +431,17 @@ struct bpf_uprobe_opts {
__u64 bpf_cookie;
/* uprobe is return probe, invoked at function return time */
bool retprobe;
+ /* name of function name or function@@LIBRARY. Partial matches
+ * work for library name, such as printf, printf@@GLIBC.
+ * To specify function entry, func_offset argument should be 0 and
+ * func_name should specify function to trace. To trace an offset
+ * within the function, specify func_name and use func_offset
+ * argument to specify argument _within_ the function.
+ */
+ const char *func_name;
size_t :0;
};
-#define bpf_uprobe_opts__last_field retprobe
+#define bpf_uprobe_opts__last_field func_name
/**
* @brief **bpf_program__attach_uprobe()** attaches a BPF program
--
1.8.3.1
