How can I verify that a program has been called by another program

Question

I have a program which should be allowed to be called by whitelisted programs only. Currently I solved the issue as such, but I wonder whether there are other more elegant solutions (maybe, by using Instructions sysvar, just like Metaplex does).

#[derive(Accounts)]
#[instruction(caller_id: Pubkey, caller_bump: u8)]
pub struct Whitelisted<'info> {
    #[account(
        seeds = [crate::SIGNATURE],
        bump = caller_bump,
        seeds::program = caller_id,
    )]
    pub caller_signature: Signer<'info>,

    #[account(
        constraint = state.can_be_called_by(caller_id),
    )]
    pub state: Account<'info, State>,
}

---snip---

pub fn whitelisted(
    ctx: Context<Whitelisted>, 
    _caller_id: Pubkey,
    _called_bump: u8,
) -> anchor_lang::Result<()> {
    ---snip---
}

And I have a caller sign caller_signature pda account with invoke_signed and also pass its address with a bump, which were stored beforehand.

Answer 1

FWIW, this is how Metaplex candy-machine-v2 does it:

use solana_program::sysvar::instructions::get_instruction_relative;

const ALLOWED_PROGRAM_ID: Pubkey = solana_program::pubkey!("...");

// in your instruction:

let current_ix = get_instruction_relative(
  0, 
  &ctx.accounts.instruction_sysvar_account.to_account_info()
).unwrap();

if !cmp_pubkeys(&current_ix.program_id, &ALLOWED_PROGRAM_ID) {
  // not my program - throw error!
}

You can get the instruction, which this CPI call is an inner instruction of, and check that it matches the program ID you expect.

This is the relevant part in candy-machine-v2 source code.