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Created tinyelf arm (markdown)

Anonymous committed 2019-05-11 18:04:10 +0000 commit d89f2206745c570eb4e68d7b6aa9c34d23d88f79

				explain/tinyelf-arm.md
			
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          +# Tiny ELF binaries on ARM

          +

          +Looks like breadbox didn't want to go down *this* rabbithole. So we'll have to do it instead.

          +

          +* [`e_machine` seems to be the only checked header field](https://code.woboq.org/linux/linux/arch/arm/kernel/elf.c.html) (`e_entry` alignment checks are normal, because if it wouldn't be aligned, the code would segfault on entry.)

          +* `phdr` parsing etc. is done architecture-independent, so the same tricks should be usable here as well.

          +  * However, on x86, we were using the way page mapping works to only have to specify a few flags, this probably can't be ported over.

          +* Each ARM opcode is 4 bytes long, and needs to be aligned. This kinda sucks for all the overlapping tricks. Also, arbitrary constants can't be loaded into registers easily.

          +  * Do we want to depend on Thumb-mode?

          +* [Dynamic linking stuff](https://linux.weeaboo.software/explain/rtld#dynamic-linking_arm)

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	1	+# Tiny ELF binaries on ARM
	2	+
	3	+Looks like breadbox didn't want to go down this rabbithole. So we'll have to do it instead.
	4	+
	5	+* [`e_machine` seems to be the only checked header field](https://code.woboq.org/linux/linux/arch/arm/kernel/elf.c.html) (`e_entry` alignment checks are normal, because if it wouldn't be aligned, the code would segfault on entry.)
	6	+* `phdr` parsing etc. is done architecture-independent, so the same tricks should be usable here as well.
	7	+ * However, on x86, we were using the way page mapping works to only have to specify a few flags, this probably can't be ported over.
	8	+* Each ARM opcode is 4 bytes long, and needs to be aligned. This kinda sucks for all the overlapping tricks. Also, arbitrary constants can't be loaded into registers easily.
	9	+ * Do we want to depend on Thumb-mode?
	10	+* [Dynamic linking stuff](https://linux.weeaboo.software/explain/rtld#dynamic-linking_arm)
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