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README.md
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README.md
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@ -176,19 +176,28 @@ https://github.com/apache/incubator-nuttx/tree/master/arch/arm64/src/common
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# NuttX Image
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TODO: Analyse NuttX Image with Ghidra and NuttX Disassembly
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Let's analyse the NuttX Image with Ghidra, to understand the NuttX Image Header and Startup Code.
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Top Part of NuttX Image Header...
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Here's the NuttX Image in Ghidra...
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Bottom Part of NuttX Image Header...
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Note that the NuttX Image jumps to `real_start` (to skip the Image Header)...
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```text
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40280000 4d 5a 00 91 add x13,x18,#0x16
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40280004 0f 00 00 14 b real_start
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```
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`real_start` is defined at 0x4028 0040...
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We see the Magic Number `ARM\x64` at offset 0x38. Searching the net for this Magic Number shows that it's actually an Arm64 Linux Kernel Header!
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We see the Magic Number `ARM\x64` at offset 0x4028 0038.
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When we refer to the NuttX Disassembly `nuttx.S`, we find happiness...
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Searching the net for this Magic Number reveals that it's actually an Arm64 Linux Kernel Header!
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When we refer to the [NuttX Arm64 Disassembly `nuttx.S`](https://github.com/lupyuen/pinephone-nuttx/releases/download/v1.0.0/nuttx.S), we find happiness...
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https://github.com/lupyuen/incubator-nuttx/blob/pinephone/arch/arm64/src/common/arm64_head.S#L79-L117
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@ -234,11 +243,11 @@ real_start:
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/* Disable all exceptions and interrupts */
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```
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NuttX Image actually follows the Linux Kernel Image Format, as defined here...
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NuttX Image actually follows the Arm64 Linux Kernel Image Format! As defined here...
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- ["Booting AArch64 Linux"](https://www.kernel.org/doc/html/latest/arm64/booting.html)
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Linux Kernel Image contains a 64-byte header as follows:
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Arm64 Linux Kernel Image contains a 64-byte header...
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```text
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u32 code0; /* Executable code */
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@ -253,7 +262,7 @@ u32 magic = 0x644d5241; /* Magic number, little endian, "ARM\x64" */
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u32 res5; /* reserved (used for PE COFF offset) */
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```
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Assuming the Start of RAM is 0x4000 0000, I wonder if this Image Load Offset might be incorrect...
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Start of RAM is 0x4000 0000. I wonder if this Image Load Offset in our NuttX Image Header might be incorrect...
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https://github.com/lupyuen/incubator-nuttx/blob/pinephone/arch/arm64/src/common/arm64_head.S#L107
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@ -261,7 +270,7 @@ https://github.com/lupyuen/incubator-nuttx/blob/pinephone/arch/arm64/src/common/
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.quad 0x480000 /* Image load offset from start of RAM */
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```
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Because the ELF Header says that the code is actually loaded at 0x4028 0000. (Instead of 0x4048 0000)
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Remember that Ghidra (and the Arm Disassembly) says that our NuttX Image is actually loaded at 0x4028 0000. (Instead of 0x4048 0000)
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RAM Size and RAM Start are defined here...
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@ -272,9 +281,9 @@ CONFIG_RAM_SIZE=134217728
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CONFIG_RAM_START=0x40000000
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```
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That's 128 MB RAM. Which should work with PinePhone's 2 GB RAM.
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That's 128 MB RAM. Which should work OK with PinePhone's 2 GB RAM.
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NuttX was built with this Linker Command, based on `make --trace`...
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The NuttX Image was built with this Linker Command, based on `make --trace`...
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```bash
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aarch64-none-elf-ld \
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@ -313,9 +322,13 @@ SECTIONS
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We'll change this to 0x4000 0000 for PinePhone, since Start of RAM is 0x4000 0000 and Image Load Offset is 0. (See below)
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We've seen the NuttX Image (which looks like a Linux Kernel Image), let's compare with a PinePhone Linux Kernel Image and see how NuttX needs to be tweaked...
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# PinePhone Image
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TODO: Disassemble a PinePhone Image with Ghidra to look at the Linux Kernel Header and Startup Code
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Let's analyse a PinePhone Linux Kernel Image with Ghidra, to look at the Linux Kernel Header and Startup Code.
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We'll use the PinePhone Jumpdrive Image, since it's small...
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https://github.com/dreemurrs-embedded/Jumpdrive
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@ -364,7 +377,15 @@ So we shift `Image` in Ghidra to start at 0x4000 0000...
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- Change Start Address to 40000000
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TODO: Verify that NuttX uses similar Header and Startup Code as PinePhone
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Note that the first instruction at 0x4000 0000 jumps to 0x4081 0000 (to skip the Linux Kernel Header)...
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```text
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40000000 00 40 20 14 b FUN_40810000
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```
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The Linux Kernel Code actually begins at 0x4081 0000...
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# TODO
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