Dynamic linking
Do your installed programs share dynamic libraries?
Findings: not really
Over half of your libraries are used by fewer than 0.1% of your executables.
Number of times each dynamic library is required by a program
libs.awk
/\t.*\.so.*/ {
n=split($1, p, "/")
split(p[n], l, ".")
lib=l[1]
if (libs[lib] == "") {
libs[lib] = 0
}
libs[lib] += 1
}
END {
for (lib in libs) {
print libs[lib] "\t" lib
}
}
Usage
$ find /usr/bin -type f -executable -print \
| xargs ldd 2>/dev/null \
| awk -f libs.awk \
| sort -rn > results.txt
$ awk '{ print NR "\t" $1 }' < results.txt > nresults.txt
$ gnuplot
gnuplot> plot 'nresults.txt'
$ find /usr/bin -type f -executable -print | wc -l 5688 $ head -n20 < results.txt 4496 libc 4484 linux-vdso 4483 ld-linux-x86-64 2654 libm 2301 libdl 2216 libpthread 1419 libgcc_s 1301 libz 1144 libstdc++ 805 liblzma 785 librt 771 libXdmcp 771 libxcb 771 libXau 755 libX11 703 libpcre 667 libglib-2 658 libffi 578 libresolv 559 libXext
Is loading dynamically linked programs faster?
Findings: definitely not
| Linkage | Avg. startup time |
|---|---|
| Dynamic | 137263 ns |
| Static | 64048 ns |
ex.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
struct timespec ts;
clock_gettime(CLOCK_MONOTONIC, &ts);
fprintf(stdout, "%ld\t", ts.tv_nsec);
fflush(stdout);
if (argc == 1) {
char *args[] = { "", "", NULL };
execvp(argv[0], args);
} else {
fprintf(stdout, "\n");
}
return 0;
}
test.sh
#!/bin/sh i=0 while [ $i -lt 1000 ] do ./ex i=$((i+1)) done
My results
$ musl-gcc -o ex ex.c
$ ./test.sh | awk 'BEGIN { sum = 0 } { sum += $2-$1 } END { print sum / NR }'
137263
$ musl-gcc -static -o ex ex.c
$ ./test.sh | awk 'BEGIN { sum = 0 } { sum += $2-$1 } END { print sum / NR }'
64048
Wouldn't statically linked executables be huge?
Findings: not really
On average, dynamically linked executables use only 4.6% of the symbols on
offer from their dependencies. A good linker will remove unused symbols.
% of symbols requested by dynamically linked programs from the libraries that it depends on
nsyms.go
package main
import (
"bufio"
"fmt"
"os"
"os/exec"
"path/filepath"
"strings"
)
func main() {
ldd := exec.Command("ldd", os.Args[1])
rc, err := ldd.StdoutPipe()
if err != nil {
panic(err)
}
ldd.Start()
var libpaths []string
scan := bufio.NewScanner(rc)
for scan.Scan() {
line := scan.Text()[1:] /* \t */
sp := strings.Split(line, " ")
var lib string
if strings.Contains(line, "=>") {
lib = sp[2]
} else {
lib = sp[0]
}
if !filepath.IsAbs(lib) {
lib = "/usr/lib/" + lib
}
libpaths = append(libpaths, lib)
}
ldd.Wait()
rc.Close()
syms := make(map[string]interface{})
for _, path := range libpaths {
objdump := exec.Command("objdump", "-T", path)
rc, err := objdump.StdoutPipe()
if err != nil {
panic(err)
}
objdump.Start()
scan := bufio.NewScanner(rc)
for i := 0; scan.Scan(); i++ {
if i < 4 {
continue
}
line := scan.Text()
sp := strings.Split(line, " ")
if len(sp) < 5 {
continue
}
sym := sp[len(sp)-1]
syms[sym] = nil
}
objdump.Wait()
rc.Close()
}
objdump := exec.Command("objdump", "-R", os.Args[1])
rc, err = objdump.StdoutPipe()
if err != nil {
panic(err)
}
objdump.Start()
used := make(map[string]interface{})
scan = bufio.NewScanner(rc)
for i := 0; scan.Scan(); i++ {
if i < 5 {
continue
}
sp := strings.Split(scan.Text(), " ")
if len(sp) < 3 {
continue
}
sym := sp[len(sp)-1]
used[sym] = nil
}
objdump.Wait()
rc.Close()
if len(syms) != 0 && len(used) != 0 && len(used) <= len(syms) {
fmt.Printf("%50s\t%d\t%d\t%f\n", os.Args[1], len(syms), len(used),
float64(len(used)) / float64(len(syms)))
}
}
Usage
$ find /usr/bin -type f -executable -print | xargs -n1 ./nsyms > results.txt
$ awk '{ n += $4 } END { print n / NR }' < results.txt
Will security vulnerabilities in libraries that have been statically linked cause large or unmanagable updates?
Findings: not really
Not including libc, the only libraries which had "critical" or "high" severity vulnerabilities in 2019 which affected over 100 binaries on my system were dbus, gnutls, cairo, libssh2, and curl. 265 binaries were affected by the rest.
The total download cost to upgrade all binaries on my system which were affected by CVEs in 2019 is 3.8 GiB. This is reduced to 1.0 GiB if you eliminate glibc.
It is also unknown if any of these vulnerabilities would have been introduced after the last build date for a given statically linked binary; if so that binary would not need to be updated. Many vulnerabilities are also limited to a specific code path or use-case, and binaries which do not invoke that code path in their dependencies will not be affected. A process to ascertain this information in the wake of a vulnerability could be automated.
extractcves.py
import email.utils
import mailbox
import re
import shlex
import time
pacman_re = re.compile(r'pacman -Syu .*')
severity_re = re.compile(r'Severity: (.*)')
mbox = mailbox.mbox("arch-security.mbox")
for m in mbox.items():
m = m[1]
date = m["Date"]
for part in m.walk():
if part.is_multipart():
continue
content_type = part.get_content_type()
[charset] = part.get_charsets("utf-8")
if content_type == 'text/plain':
body = part.get_payload(decode=True).decode(charset)
break
pkgs = pacman_re.findall(body)
severity = severity_re.findall(body)
date = email.utils.parsedate(date)
if len(pkgs) == 0 or date is None:
continue
if date[0] <= 2018 or date[0] > 2019:
continue
severity = severity[0]
args = shlex.split(pkgs[0])
pkg = args[2].split(">=")[0]
print(pkg, severity)
$ python3 extractcves.py | grep Critical > cves.txt
$ xargs pacman -Ql < cves.txt | grep \\.so | awk '{print $1}' | sort -u>affected.txt
# Manually remove packages like Firefox, Thunderbird, etc; write remainder.txt
$ xargs pacman -Ql < remainder.txt | grep '/usr/lib/.*.so$' | awk '{ print $2 }' > libs.txt
$ ldd /usr/bin/* >ldd.txt
$ ./scope.sh <libs.txt | sort -nr >sobjects.txt
sobjects.txt is a sorted list of shared objects and the number of executables that link to them. To find the total size of affected binaries, I ran the following command:
# With libc $ egrep -la 'libc.so|libm.so|libdl.so|libpthread.so|librt.so|libresolv.so|libdbus-1.so|libgnutls.so|libcairo.so|libutil.so|libssh2.so|libcurl.so|libcairo-gobject.so|libcrypt.so|libspice-server.so|libarchive.so|libSDL2-2.0.so|libmvec.so|libmagic.so|libtextstyle.so|libgettextlib-0.20.2.so|libgettextsrc-0.20.2.so|libMagickWand-7.Q16HDRI.so|libMagickCore-7.Q16HDRI.so|libbfd-2.34.0.so|libpolkit-gobject-1.so|libwebkit2gtk-4.0.so|libjavascriptcoregtk-4.0.so|libpolkit-agent-1.so|libgs.so|libctf.so|libSDL.so|libopcodes-2.34.0.so|libQt5WebEngine.so|libQt5WebEngineCore.so|libctf-nobfd.so|libcairo-script-interpreter.so' /usr/bin/* | xargs wc -c # Without libc $ egrep -la 'libdbus-1.so|libgnutls.so|libcairo.so|libssh2.so|libcurl.so|libcairo-gobject.so|libcrypt.so|libspice-server.so|libarchive.so|libSDL2-2.0.so|libmvec.so|libmagic.so|libtextstyle.so|libgettextlib-0.20.2.so|libgettextsrc-0.20.2.so|libMagickWand-7.Q16HDRI.so|libMagickCore-7.Q16HDRI.so|libbfd-2.34.0.so|libpolkit-gobject-1.so|libwebkit2gtk-4.0.so|libjavascriptcoregtk-4.0.so|libpolkit-agent-1.so|libgs.so|libctf.so|libSDL.so|libopcodes-2.34.0.so|libQt5WebEngine.so|libQt5WebEngineCore.so|libctf-nobfd.so|libcairo-script-interpreter.so' /usr/bin/* | xargs wc -c
Doesn't static linking prevent ASLR from working?
No.
We've had ASLR for statically linked binaries for some time now. It's called static PIE.
Test environment
- Arch Linux, up-to-date as of 2020-06-25
- 2188 packages installed
- gcc 10.1.0