Script/Tool predicate for ARM ELF compiled for Thumb OR Arm

Question

I have rootfs and klibc file systems. I am creating make rules and some developers have an older compiler without inter-netwo

Answer 1

Since thumb and arm sequences can be freely interchanged within an object file, even within the same section, plain ELF header inspection is not going to help you whether a file includes Thumb instructions or not.

A slightly roundabout and still not 100% foolproof way would be to use readelf -r and check if the output contains "R_ARM_THM", indicating a relocation for thumb.

Answer 2

The readelf -A output doesn't describe the elf contents. It just describes the capabilities of the processor and or system that is expected or feed to the compiler. As I have an ARM926 CPU which is an ARMV5TEJ processor, gcc/ld will always set Tag_THUMB_ISA_use: Thumb-1 as it just means that ARMV5TEJ is recognized as being Thumb-1 capable. It says nothing about the code itself.

Examining the Linux arch/arm/kernel/elf.c routine elf_check_arch() shows a check for x->e_entry & 1. This leads to the following script,

readelf -h $1 | grep -q Entry.*[13579bdf]$

Ie, just look at the initial ELF entry value and see if the low bit is set. This is a fast check that fits the spirit of what I am looking for. unixsmurf has a good point that the code inside any ELF can mix and match ARM and Thumb. This maybe ok, if the program dynamically ids the CPU and selects an appropriate routine. Ie, just the presence of a Thumb instruction doesn't mean that code will execute.

Just looking at the entry value does determine which gcc compiler flags were used, at least for gcc versions 4.6 to 4.7.