.NET 10 Android runtime libraries ship without .eh_frame, truncating native crash stacks to 1-2 frames

[chriswindram]

Android framework version

net10.0-android

Affected platform version

.NET 10, Microsoft.Android.Runtime.Mono.36.android-arm64 and Microsoft.Android.Runtime.CoreCLR.36.android-arm64, both 36.1.69, android-arm64

Description

Summary

The .NET 10 Android runtime libraries contain no unwind information at all — no .eh_frame, no
.eh_frame_hdr, and no PT_GNU_EH_FRAME program header. The equivalent .NET 9 library has all three.

Consequently nothing can unwind out of these libraries. Google Play tombstones for a crash inside the
runtime stop at frame #00 or #01, and no caller is recoverable. Uploading debug symbols does not
help, because what is missing is CFI, not symbol names.

Found whilst investigating #12696

Evidence

net9  Runtime.35 / 35.0.61          PT_GNU_EH_FRAME=YES  sections: .eh_frame_hdr, .eh_frame
net10 Runtime.Mono.36 / 36.1.69     PT_GNU_EH_FRAME=NO   sections: NONE
net10 Runtime.CoreCLR.36 / 36.1.69  PT_GNU_EH_FRAME=NO   sections: NONE

Verified independently from both the section header table and the program header table. .symtab is
still present in all three (1,430 symbols on net9, 1,594 on net10 Mono), so function names still
resolve — only the ability to walk the stack is gone.

Impact

A real example. We have a production crash inside the runtime affecting ~1.6% of sessions. The best
of three Play tombstones has two frames; the other two have only frame #00. The calling function is
unrecoverable from any crash report, so the crash could not be attributed to a subsystem at all.

We eventually identified the caller by statically scanning .text for BL instructions targeting the
symbol and mapping call sites back to .symtab. That worked, but it should not be the supported route
for diagnosing a crash in a shipped runtime.

This affects anyone triaging a native crash in a .NET 10 Android app, and it silently degrades Play
vitals, Sentry and Crashlytics alike — none of them can do anything about it, because the information
is absent from the binary.

Possibly related

--gc-sections and section-splitting work for Android native release builds landed in this window and
may be the cause, though we have not confirmed that. Whatever the cause, .eh_frame ought to survive
it: --gc-sections alone does not normally remove unwind tables, so this may be an unintended
consequence of an additional strip step, or of -fno-asynchronous-unwind-tables.

Ask

Ship .eh_frame/.eh_frame_hdr in the release runtime libraries, as .NET 9 did. The size cost is
modest relative to making every native crash inside the runtime undiagnosable.

Steps to Reproduce

No app is required. Against the installed workload packs:

readelf -lS <pack>/runtimes/android-arm64/native/libmono-android.release.so | grep -E "eh_frame|GNU_EH_FRAME"

1. Run it against Microsoft.Android.Runtime.35 / 35.0.61 — .eh_frame, .eh_frame_hdr and
   PT_GNU_EH_FRAME are all present.
2. Run it against Microsoft.Android.Runtime.Mono.36 / 36.1.69 — all three are absent.
3. Same for Microsoft.Android.Runtime.CoreCLR.36 / 36.1.69 (libnet-android.release.so).

To see the consequence, take any native crash inside the runtime on a .NET 10 build and read the
tombstone in Play Console: it will stop at frame #00 or #1.

Did you find any workaround?

None for the missing unwind tables themselves.

The only route we found to a caller was static analysis of the shipped binary: locate the faulting
symbol in .symtab, scan .text for AArch64 BL instructions (top 6 bits 0b100101, sign-extended
imm26 x 4) whose target is that symbol, and map each call site back to its containing function. That
recovers callers without unwinding, but only direct calls, and it cannot recover a runtime call stack.

Relevant log output

Section and program header comparison across the three packs:

  net9  Runtime.35 / 35.0.61          PT_GNU_EH_FRAME=YES  unwind sections: .eh_frame_hdr, .eh_frame
  net10 Runtime.Mono.36 / 36.1.69     PT_GNU_EH_FRAME=NO   unwind sections: NONE
  net10 Runtime.CoreCLR.36 / 36.1.69  PT_GNU_EH_FRAME=NO   unwind sections: NONE

A representative truncated tombstone from Google Play (this is the FULL backtrace as reported):

  pid: 0, tid: 23507  >>> com.example.app <<<

  backtrace:
    #00  pc 0x0000000000069c4c  /apex/com.android.runtime/lib64/bionic/libc.so (__memcmp_aarch64+12)
    #01  pc 0x00000000000afa54  .../split_config.arm64_v8a.apk!libmono-android.release.so
         (...robin_hash<...>::find_impl<...>(...) const+208)
         (BuildId: ae7ad8820dd75996ecdee692e377f8586298ceb0)

Two of the three sample reports for the same crash contain only frame #00.