Verify: sysvar_rent_acc's key to ensure that the account passed is a valid sysvar rent account

[DhruvWebDev]

Solana uses accounts for everything. To store data for your programs. To implement feature gates. To store programs themselves. And also to provide environment information about the cluster to your programs! This is done through special accounts, so called sysvars. These accounts have special addresses starting with Sysvar, such as SysvarC1ock11111111111111111111111111111111. Loading them into your instruction allows your program to access their information, such as the current slot or timestamp from the clock sysvar. Other sysvars include the EpochSchedule, Fees, Instructions, RecentBlockhashes, Rent, Slothashes, SlotHistory, StakeHistory, EpochRewards and LastRestartSlot. All of those sysvars are useful for different things, ranging from calculating rent costs and timestamps to getting information about validator emissions, and transaction introspection. The most used ones are probably the Rent, Clock, and Instructions sysvars, and every dev should know how to use them. Since simd 0127, which was activated just a few weeks ago, sysvars don't have to be included as accounts in the instruction as they can be accessed through syscalls directly. Though programs can still access them through accounts if they want to. In practice, you can simply access your sysvars by using their get method like let clock = Clock::get()?; In the past, we've seen multiple hacks where sysvar account addresses weren't checked by a program and a hacker was able to provide a fake sysvar account. So make sure to check the sysvar address you're using - or even better to use the syscall interface instead of accounts these days!