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to extend on it, _start is where .text begins and address of that is set by linker

The entry point can be anywhere in the .text section, and often won't be at the beginning of the section.

Technically it doesn't even need to be in the .text section, it could be anywhere in the address space. You'll get a segfault if it's not somewhere executable though (assuming you're on a system with an appropriately configured MMU)

yes and then you'll have a bad time, but at the same time per convention _start is where .text begins. You can see where it starts with readelf --file-header <executable> and look at Entry point address field. You can change it, yes.

A common hack to reduce ELF size is actually to start the first section (possibly the .text) right on the elf header, as this circumvents the alignment requirements.

probably not even mandatory.. lots of /usr/bin stuff on my ubuntu machine have __libc_start_main only

No, it's not even a convention, _start is most commonly not where .text begins.

Compiling a static hello world binary on my system (aarch64 fedora 39, gcc -static hello.c -o hello), .text starts at 0x410080, e_entry is at 0x4103c0, and the _start symbol is also at 0x4103c0. This is not unusual at all.

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