runtime: make static/dynamic startup detection work with musl on ppc64le

The glibc loader explicitly sets the first doubleword on the stack (R1)
to $0 to indicate it was dynamically loaded.

An ELFv2 ABI compliant loader will set R3/R4 to argc/argv when starting
the process, and R13 to TLS. musl is not compliant. Instead it passes
argc/argv like the kernel, but R3/R4 are in an undefined state and R13
is valid.

With the knowledge above, the startup code can be modified to
dynamically handle all three cases when linked internally.

Fixes #51787

Change-Id: I5de33862c161900d9161817388bbc13a65fdc69c
Reviewed-on: https://go-review.googlesource.com/c/go/+/394654
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Paul Murphy <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Trust: Paul Murphy <[email protected]>
Trust: Lynn Boger <[email protected]>

src/runtime/rt0_linux_ppc64le.s

@@ -147,25 +147,35 @@ TEXT _main<>(SB),NOSPLIT,$-8
 	// In a statically linked binary, the stack contains argc,
 	// argv as argc string pointers followed by a NULL, envv as a
 	// sequence of string pointers followed by a NULL, and auxv.
-	// There is no TLS base pointer.
+	// The TLS pointer should be initialized to 0.
 	//
-	// In a dynamically linked binary, r3 contains argc, r4
-	// contains argv, r5 contains envp, r6 contains auxv, and r13
+	// In an ELFv2 compliant dynamically linked binary, R3 contains argc,
+	// R4 contains argv, R5 contains envp, R6 contains auxv, and R13
 	// contains the TLS pointer.
 	//
-	// Figure out which case this is by looking at r4: if it's 0,
-	// we're statically linked; otherwise we're dynamically
-	// linked.
-	CMP	R0, R4
-	BNE	dlink
-
-	// Statically linked
+	// When loading via glibc, the first doubleword on the stack points
+	// to NULL a value. (that is *(uintptr)(R1) == 0). This is used to
+	// differentiate static vs dynamicly linked binaries.
+	//
+	// If loading with the musl loader, it doesn't follow the ELFv2 ABI. It
+	// passes argc/argv similar to the linux kernel, R13 (TLS) is
+	// initialized, and R3/R4 are undefined.
+	MOVD	(R1), R12
+	CMP	R0, R12
+	BEQ	tls_and_argcv_in_reg
+
+	// Arguments are passed via the stack (musl loader or a static binary)
 	MOVD	0(R1), R3 // argc
 	ADD	$8, R1, R4 // argv
+
+	// Did the TLS pointer get set? If so, don't change it (e.g musl).
+	CMP	R0, R13
+	BNE	tls_and_argcv_in_reg
+
 	MOVD	$runtime·m0+m_tls(SB), R13 // TLS
 	ADD	$0x7000, R13
 
-dlink:
+tls_and_argcv_in_reg:
 	BR	main(SB)
 
 TEXT main(SB),NOSPLIT,$-8