[android] arm64 running on x64 emulator unexpected FPSIMD_MAGIC assert

[lambdageek]

Recent versions (API 30 x86_64) of the Android emultator can run arm64 binaries on an x86_64 emultator using some kind of binary translation. When an app is running in this environment, it looks like the struct sigcontext __reserved field is not laid out with the fpsimd_context first.

As a result this code asserts:

runtime/src/mono/mono/utils/mono-context.c

Lines 536 to 539 in b1b7ad4

	struct fpsimd_context *fpctx = (struct fpsimd_context*)&((ucontext_t*)sigctx)->uc_mcontext.__reserved;
	int i;
	g_assert (fpctx->head.magic == FPSIMD_MAGIC);

Linux kernel sigcontext __reserved note

Instead of looking at the first context in __reserved and asserting that it has FPSIMD_MAGIC, we should check head.magic == FPSIMD_MAGIC and if not, move forward by head.size bytes and try again until we see head.magic == 0 (the last block).

Potentially we can assume that the offset won't change between calls to mono_sigctx_to_monoctx and cache the pre-computed offset (and assert that the magic is the right one).

---

Reproduction steps for XA:

1. Turn off fast deployment by adding <EmbedAssembliesIntoApk>true</EmbedAssembliesIntoApk> to the csproj file
2. Remove definition for $(AndroidSupportedAbis) if there is any in the .csproj file
3. Add <RuntimeIdentifiers>android-arm;android-arm64</RuntimeIdentifiers> to the csproj file
4. Build and run the app in Release mode on x86_64 emulator crashes using dotnet build -c Release -t:Run

Tagging subscribers to 'arch-android': @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Recent versions (API 30 x86_64) of the Android emultator can run arm64 binaries on an x86_64 emultator using some kind of binary translation. When an app is running in this environment, it looks like the struct sigcontext __reserved field is not laid out with the fpsimd_context first.

As a result this code asserts:

runtime/src/mono/mono/utils/mono-context.c

Lines 536 to 539 in b1b7ad4

	struct fpsimd_context *fpctx = (struct fpsimd_context*)&((ucontext_t*)sigctx)->uc_mcontext.__reserved;
	int i;
	g_assert (fpctx->head.magic == FPSIMD_MAGIC);

Instead of looking at the first context in __reserved and asserting that it has FPSIMD_MAGIC, we should check head.magic == FPSIMD_MAGIC and if not, move forward by head.size bytes and try again until we see head.magic == 0 (the last block).

Potentially we can assume that the offset won't change between calls to mono_sigctx_to_monoctx and cache the pre-computed offset (and assert that the magic is the right one).

---

Reproduction steps for XA:

1. Turn off fast deployment
2. $(AndroidSupportedAbis) is blank, just not in the .csproj file
3. Defaults to armeabi-v7a;arm64-v8a
4. Running on x86_64 emulator crashes

Author:	lambdageek
Assignees:	-
Labels:	arch-arm64, os-android, untriaged, area-VM-meta-mono
Milestone:	-

[lambdageek]

@jonathanpeppers can you verify/update the repro steps

/cc @grendello

[jonathanpeppers]

• Build & debug this app on an x86_64 API 30 emulator: App34.zip

I think as long as you're using VS 2022, the issue should repro.

To workaround, you can either go to Project properties | Android Options | Advanced, and enable all ABIs:

Or Release builds also happen to work, not sure why.

[akoeplinger]

Moving to 7.0