uprobe_helpers.c

// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
/* Copyright (c) 2021 Google LLC. */
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
#include <gelf.h>

#define warn(...) fprintf(stderr, __VA_ARGS__)

/*
 * Returns 0 on success; -1 on failure.  On sucess, returns via `path` the full
 * path to the program for pid.
 */
int get_pid_binary_path(pid_t pid, char *path, size_t path_sz)
{
	ssize_t ret;
	char proc_pid_exe[32];

	if (snprintf(proc_pid_exe, sizeof(proc_pid_exe), "/proc/%d/exe", pid)
	    >= sizeof(proc_pid_exe)) {
		warn("snprintf /proc/PID/exe failed");
		return -1;
	}
	ret = readlink(proc_pid_exe, path, path_sz);
	if (ret < 0) {
		warn("No such pid %d\n", pid);
		return -1;
	}
	if (ret >= path_sz) {
		warn("readlink truncation");
		return -1;
	}
	path[ret] = '\0';

	return 0;
}

/*
 * Returns 0 on success; -1 on failure.  On success, returns via `path` the full
 * path to a library matching the name `lib` that is loaded into pid's address
 * space.
 */
int get_pid_lib_path(pid_t pid, const char *lib, char *path, size_t path_sz)
{
	FILE *maps;
	char *p;
	char proc_pid_maps[32];
	char line_buf[1024];

	if (snprintf(proc_pid_maps, sizeof(proc_pid_maps), "/proc/%d/maps", pid)
	    >= sizeof(proc_pid_maps)) {
		warn("snprintf /proc/PID/maps failed");
		return -1;
	}
	maps = fopen(proc_pid_maps, "r");
	if (!maps) {
		warn("No such pid %d\n", pid);
		return -1;
	}
	while (fgets(line_buf, sizeof(line_buf), maps)) {
		if (sscanf(line_buf, "%*x-%*x %*s %*x %*s %*u %s", path) != 1)
			continue;
		/* e.g. /usr/lib/x86_64-linux-gnu/libc-2.31.so */
		p = strrchr(path, '/');
		if (!p)
			continue;
		if (strncmp(p, "/lib", 4))
			continue;
		p += 4;
		if (strncmp(lib, p, strlen(lib)))
			continue;
		p += strlen(lib);
		/* libraries can have - or . after the name */
		if (*p != '.' && *p != '-')
			continue;

		fclose(maps);
		return 0;
	}

	warn("Cannot find library %s\n", lib);
	fclose(maps);
	return -1;
}

/*
 * Returns 0 on success; -1 on failure.  On success, returns via `path` the full
 * path to the program.
 */
static int which_program(const char *prog, char *path, size_t path_sz)
{
	FILE *which;
	char cmd[100];

	if (snprintf(cmd, sizeof(cmd), "which %s", prog) >= sizeof(cmd)) {
		warn("snprintf which prog failed");
		return -1;
	}
	which = popen(cmd, "r");
	if (!which) {
		warn("which failed");
		return -1;
	}
	if (!fgets(path, path_sz, which)) {
		warn("fgets which failed");
		pclose(which);
		return -1;
	}
	/* which has a \n at the end of the string */
	path[strlen(path) - 1] = '\0';
	pclose(which);
	return 0;
}

/*
 * Returns 0 on success; -1 on failure.  On success, returns via `path` the full
 * path to the binary for the given pid.
 * 1) pid == x, binary == ""    : returns the path to x's program
 * 2) pid == x, binary == "foo" : returns the path to libfoo linked in x
 * 3) pid == 0, binary == ""    : failure: need a pid or a binary
 * 4) pid == 0, binary == "bar" : returns the path to `which bar`
 *
 * For case 4), ideally we'd like to search for libbar too, but we don't support
 * that yet.
 */
int resolve_binary_path(const char *binary, pid_t pid, char *path, size_t path_sz)
{
	if (!strcmp(binary, "")) {
		if (!pid) {
			warn("Uprobes need a pid or a binary\n");
			return -1;
		}
		return get_pid_binary_path(pid, path, path_sz);
	}
	if (pid)
		return get_pid_lib_path(pid, binary, path, path_sz);

	if (which_program(binary, path, path_sz)) {
		/*
		 * If the user is tracing a program by name, we can find it.
		 * But we can't find a library by name yet.  We'd need to parse
		 * ld.so.cache or something similar.
		 */
		warn("Can't find %s (Need a PID if this is a library)\n", binary);
		return -1;
	}
	return 0;
}

/*
 * Opens an elf at `path` of kind ELF_K_ELF.  Returns NULL on failure.  On
 * success, close with close_elf(e, fd_close).
 */
static Elf *open_elf(const char *path, int *fd_close)
{
	int fd;
	Elf *e;

	if (elf_version(EV_CURRENT) == EV_NONE) {
		warn("elf init failed\n");
		return NULL;
	}
	fd = open(path, O_RDONLY);
	if (fd < 0) {
		warn("Could not open %s\n", path);
		return NULL;
	}
	e = elf_begin(fd, ELF_C_READ, NULL);
	if (!e) {
		warn("elf_begin failed: %s\n", elf_errmsg(-1));
		close(fd);
		return NULL;
	}
	if (elf_kind(e) != ELF_K_ELF) {
		warn("elf kind %d is not ELF_K_ELF\n", elf_kind(e));
		elf_end(e);
		close(fd);
		return NULL;
	}
	*fd_close = fd;
	return e;
}

static void close_elf(Elf *e, int fd_close)
{
	elf_end(e);
	close(fd_close);
}

/* Returns the offset of a function in the elf file `path`, or -1 on failure. */
off_t get_elf_func_offset(const char *path, const char *func)
{
	off_t ret = -1;
	int i, fd = -1;
	Elf *e;
	Elf_Scn *scn;
	Elf_Data *data;
	GElf_Shdr shdr[1];
	GElf_Sym sym[1];
	size_t shstrndx;
	char *n;

	e = open_elf(path, &fd);

	if (elf_getshdrstrndx(e, &shstrndx) != 0)
		goto out;

	scn = NULL;
	while ((scn = elf_nextscn(e, scn))) {
		if (!gelf_getshdr(scn, shdr))
			continue;
		if (!(shdr->sh_type == SHT_SYMTAB || shdr->sh_type == SHT_DYNSYM))
			continue;
		data = NULL;
		while ((data = elf_getdata(scn, data))) {
			for (i = 0; gelf_getsym(data, i, sym); i++) {
				n = elf_strptr(e, shdr->sh_link, sym->st_name);
				if (!n)
					continue;
				if (GELF_ST_TYPE(sym->st_info) != STT_FUNC)
					continue;
				if (!strcmp(n, func)) {
					ret = sym->st_value;
					goto out;
				}
			}
		}
	}

out:
	close_elf(e, fd);
	return ret;
}