kexec: derive purgatory entry from symbol

In the Linux kernel, the following vulnerability has been resolved:

kexec: derive purgatory entry from symbol

kexec_load_purgatory() derives image->start by locating e_entry inside an

SHF_EXECINSTR section. If the purgatory object contains multiple

executable sections with overlapping sh_addr, the entrypoint check can

match more than once and trigger a WARN.

Derive the entry section from the purgatory_start symbol when present and

compute image->start from its final placement. Keep the existing e_entry

fallback for purgatories that do not expose the symbol.

WARNING: kernel/kexec_file.c:1009 at kexec_load_purgatory+0x395/0x3c0, CPU#10: kexec/1784

Call Trace:

<TASK>

bzImage64_load+0x133/0xa00

__do_sys_kexec_file_load+0x2b3/0x5c0

do_syscall_64+0x81/0x610

entry_SYSCALL_64_after_hwframe+0x76/0x7e

[[email protected]: move helper to avoid forward declaration, per Baoquan]