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I recently heard about the esp trick: some packers push all registers on the stack, and when unpacking is done, they are restored. Placing a hardware breakpoint on esp we can stop there and get the original entry point.

Why is it necessary to use a hardware breakpoint? This site argues that because software breakpoints modify the code. But why is that a problem here?

Every example I met uses Ollydbg. How can I set such a breakpoint in gdb?

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Placing a hardware breakpoint on esp we can stop there and get the original entry point.

Why is it necessary to use a hardware breakpoint?

The value of ESP is an address on the stack. The data at that memory address may get read or written, but won't get executed¹ since it's not code. Software breakpoints are only useful on code that gets executed, and since the data at that memory address won't get executed, a software breakpoint won't be helpful.

¹ There are exceptions to this, but it's out of context for your question.

How can I set such a breakpoint in gdb?

In gdb, you can set a hardware breakpoint on the memory address pointed to by ESP by setting a watchpoint, which is documented here.

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