[PATCH v1 0/7] DCE/DSE: Add Dead Syscalls Elimination support, part1

[Yuan Tan]

I didn't test DSE with explicit KEEP() in the previous mail. So, I will make up
for it now.

This test result is about DEAD_CODE_DATA_ELIMINATION (DCE) and dead syscalls
elimination (DSE). It's based on config[1] and a simple hello.c initramfs.

We set CONFIG_SYSCALLS_USED="sys_write sys_exit sys_reboot", which is used by
hello.c to simply print "Hello" then exit and shut down qemu.

|                                                              | syscall remain | vmlinux size     | vmlinux after strip |
| ------------------------------------------------------------ | -------------- | ---------------- | ------------------- |
| disable DCE                                                  | 236            | 2559632          | 1963400             |
| enable DCE                                                   | 208            | 2037384 (-20.4%) | 1485776 (-24.3%)    |
| enable DCE and DSE with explicit KEEP() of except table      | 17             | 1899208 (-25.8%) | 1387272 (-29.3%)    |
| enable DCE and DSE without KEEP() (By SHF_GROUP method)      | 3              | 1856640 (-27.6%) | 1354424 (-31.0%)    |
| enable DCE and DSE without KEEP() (By SHE_LINK_ORDER method) | 3              | 1856664 (-27.6%) | 1354424 (-31.0%)    |


It shows that dead syscalls elimination can save 7% of space based on DCE.

Although no KEEP() can only save up 2% space, it can reduce the attack surface
and eliminate the misuse of KEEP(). It ensures that every orphan section is not
orphaned anymore.

[1]: https://pastebin.com/KG4fd7aT