Fix sceNetHtonl/Htons/Ntohl/Ntohs discarding their converted result (#211)

Each of the four byte-order helpers computed the swap into Rax and then
returned via ctx.SetReturn(0). SetReturn writes its argument into Rax, so it
immediately overwrote the converted value with 0 and the guest saw every
sceNetHtonl / sceNetHtons / sceNetNtohl / sceNetNtohs call return 0.

Leave the swapped value in Rax and return ORBIS_GEN2_OK as the dispatch status
instead, matching how the value-returning exports in this file (e.g.
sceNetPoolCreate) already work.

Add a NetExports test suite covering the swaps, the 16-bit width masking, an
htonl/ntohl round-trip, and a regression guard that a non-zero input never
converts to 0.

src/SharpEmu.Libs/Network/NetExports.cs

@@ -344,8 +344,10 @@ public static class NetExports
     public static int NetHtonl(CpuContext ctx)
     {
         var value = unchecked((uint)ctx[CpuRegister.Rdi]);
+        // The byte-swapped result is the return value and already lives in Rax; return OK as the
+        // dispatch status without going through SetReturn, which would overwrite Rax with 0.
         ctx[CpuRegister.Rax] = BinaryPrimitives.ReverseEndianness(value);
-        return ctx.SetReturn(0);
+        return (int)OrbisGen2Result.ORBIS_GEN2_OK;
     }
 
     [SysAbiExport(
@@ -356,8 +358,10 @@ public static class NetExports
     public static int NetHtons(CpuContext ctx)
     {
         var value = unchecked((ushort)ctx[CpuRegister.Rdi]);
+        // The byte-swapped result is the return value and already lives in Rax; return OK as the
+        // dispatch status without going through SetReturn, which would overwrite Rax with 0.
         ctx[CpuRegister.Rax] = BinaryPrimitives.ReverseEndianness(value);
-        return ctx.SetReturn(0);
+        return (int)OrbisGen2Result.ORBIS_GEN2_OK;
     }
 
     [SysAbiExport(
@@ -368,8 +372,10 @@ public static class NetExports
     public static int NetNtohl(CpuContext ctx)
     {
         var value = unchecked((uint)ctx[CpuRegister.Rdi]);
+        // The byte-swapped result is the return value and already lives in Rax; return OK as the
+        // dispatch status without going through SetReturn, which would overwrite Rax with 0.
         ctx[CpuRegister.Rax] = BinaryPrimitives.ReverseEndianness(value);
-        return ctx.SetReturn(0);
+        return (int)OrbisGen2Result.ORBIS_GEN2_OK;
     }
 
     [SysAbiExport(
@@ -380,8 +386,10 @@ public static class NetExports
     public static int NetNtohs(CpuContext ctx)
     {
         var value = unchecked((ushort)ctx[CpuRegister.Rdi]);
+        // The byte-swapped result is the return value and already lives in Rax; return OK as the
+        // dispatch status without going through SetReturn, which would overwrite Rax with 0.
         ctx[CpuRegister.Rax] = BinaryPrimitives.ReverseEndianness(value);
-        return ctx.SetReturn(0);
+        return (int)OrbisGen2Result.ORBIS_GEN2_OK;
     }
 
     [SysAbiExport(

tests/SharpEmu.Libs.Tests/Network/NetExportsTests.cs

@@ -0,0 +1,94 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.HLE;
+using SharpEmu.Libs.Network;
+using Xunit;
+
+namespace SharpEmu.Libs.Tests.Network;
+
+// The libSceNet byte-order helpers take their operand in Rdi and return the converted value in
+// Rax. They swap endianness unconditionally, which is correct on the little-endian hosts (and
+// little-endian guest) the emulator targets, so network (big-endian) order is always a byte swap.
+public sealed class NetExportsTests
+{
+    private readonly CpuContext _ctx = new(new FakeCpuMemory(0x1_0000_0000, 0x1000), Generation.Gen5);
+
+    [Fact]
+    public void Htonl_SwapsAllFourBytes()
+    {
+        _ctx[CpuRegister.Rdi] = 0x01020304;
+
+        Assert.Equal(0, NetExports.NetHtonl(_ctx));
+        Assert.Equal(0x04030201UL, _ctx[CpuRegister.Rax]);
+    }
+
+    [Fact]
+    public void Ntohl_SwapsAllFourBytes()
+    {
+        _ctx[CpuRegister.Rdi] = 0x01020304;
+
+        Assert.Equal(0, NetExports.NetNtohl(_ctx));
+        Assert.Equal(0x04030201UL, _ctx[CpuRegister.Rax]);
+    }
+
+    [Fact]
+    public void Htons_SwapsLowTwoBytesOnly()
+    {
+        // High bits above the 16-bit short must be ignored, not folded into the result.
+        _ctx[CpuRegister.Rdi] = 0xFFFF_0102;
+
+        Assert.Equal(0, NetExports.NetHtons(_ctx));
+        Assert.Equal(0x0201UL, _ctx[CpuRegister.Rax]);
+    }
+
+    [Fact]
+    public void Ntohs_SwapsLowTwoBytesOnly()
+    {
+        _ctx[CpuRegister.Rdi] = 0xFFFF_0102;
+
+        Assert.Equal(0, NetExports.NetNtohs(_ctx));
+        Assert.Equal(0x0201UL, _ctx[CpuRegister.Rax]);
+    }
+
+    [Fact]
+    public void Htonl_IgnoresBitsAboveThe32BitWord()
+    {
+        _ctx[CpuRegister.Rdi] = 0xDEADBEEF_01020304;
+
+        Assert.Equal(0, NetExports.NetHtonl(_ctx));
+        Assert.Equal(0x04030201UL, _ctx[CpuRegister.Rax]);
+    }
+
+    [Theory]
+    [InlineData(0xDEADBEEFUL)]
+    [InlineData(0x00000000UL)]
+    [InlineData(0xFFFFFFFFUL)]
+    [InlineData(0x00000001UL)]
+    public void HtonlThenNtohl_RoundTripsToOriginal(ulong value)
+    {
+        _ctx[CpuRegister.Rdi] = value;
+        NetExports.NetHtonl(_ctx);
+
+        _ctx[CpuRegister.Rdi] = _ctx[CpuRegister.Rax];
+        NetExports.NetNtohl(_ctx);
+
+        Assert.Equal(value, _ctx[CpuRegister.Rax]);
+    }
+
+    // Regression guard: a non-palindromic value must not come back as 0. The functions previously
+    // computed the swap into Rax and then called SetReturn(0), which overwrote Rax, so every
+    // sceNetHtonl/Htons/Ntohl/Ntohs call returned 0 regardless of input.
+    [Fact]
+    public void ByteOrderConversions_DoNotReturnZeroForNonZeroInput()
+    {
+        _ctx[CpuRegister.Rdi] = 0x01020304;
+        NetExports.NetHtonl(_ctx);
+        Assert.NotEqual(0UL, _ctx[CpuRegister.Rax]);
+
+        _ctx[CpuRegister.Rax] = 0;
+        _ctx[CpuRegister.Rdi] = 0x0102;
+        NetExports.NetHtons(_ctx);
+        Assert.NotEqual(0UL, _ctx[CpuRegister.Rax]);
+    }
+}