[Bug dynamic-link/31959] New: Feature request: special static-pie capable of loading the interpreter from a relative path

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=31959

            Bug ID: 31959
           Summary: Feature request: special static-pie capable of loading
                    the interpreter from a relative path
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

Many groups want to distribute dynamically-linked ELF executables using new
libc/rtld features (e.g. RELR, AArch64 PAuth), or simply because they don't
want to set up a build environment with a very old version of glibc.
https://www.corsix.org/content/for-want-of-a-relative-path is a nice write-up
on this topic.
Traditionally, this required a separate launcher script if binary patching is
avoided.

    #!/usr/bin/env bash

    ORIGIN="$(dirname "$(readlink -f "$0")")"
    exec "$ORIGIN/libs/ld-linux-x86-64.so.2" --library-path
"$ORIGIN/libs:$LD_LIBRARY_PATH" "$ORIGIN/my_executable" "$@"

corsix.org notes a few drawbacks:

    * Execing the dynamic linker is a slightly obscure feature, which can
increase the chance of hitting bugs (e.g. BZ#16381, BZ#24900).
    * If my_executable tries to open("/proc/self/exe") or
readlink("/proc/self/exe") or similar, it'll get ld-linux-x86-64.so.2 rather
than itself.
    * Having a launcher script is aesthetically displeasing.

In addition, /proc/*/exe and proc/*/cmdline would look different and there
might be weird re-exec issues.
gcompat (from Adélie Linux) developers should know more caveats.

---

https://github.com/rcombs/musl/tree/portable-dynamic solves the "relative
interpreter problem" by building a special static-pie (ET_DSO, no PT_INTERP)
that links in a custom loader (dcrt1.o, which replaces rcrt1.o).
The static-pie mmap's ld-musl, adjusts auxv to point to it, and then jumps to
ld-musl's entry point.
(See https://www.openwall.com/lists/musl/2022/09/26/13 for some public
discussions.)

---

There's a growing interest in using new dynamic linker features for ELF
executables. However, a solution hasn't yet been widely adopted by major libc
implementations.

I imagine a future where glibc supports a custom loader capable of loading the
interpreter from a relative path.
This would provide a clean and well-supported solution, addressing the needs of
many developers.

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