brk (nr 12)

Linux Signature

int brk(void *addr);

Note: The raw syscall returns the new program break on success (not 0 like the glibc wrapper).

Description

Sets the end of the process’s data segment (the “program break”). Increasing the break allocates memory; decreasing it deallocates.

Current Implementation

• brk(0) or brk(addr < brk_base): Returns the current program break without modification. This is how musl queries the initial break.
• brk(addr <= brk_current): Shrinks the break. Updates brk_current but does not unmap or free any pages.
• brk(addr > brk_current): Grows the break. The requested address is page-aligned up. For each new page:
  • A physical frame is allocated via alloc_dma_pages(1).
  • The frame is zeroed.
  • The frame is mapped into the process’s page table with PRESENT | WRITABLE | USER_ACCESSIBLE | NO_EXECUTE.
  • brk_current is updated to the new page-aligned address.
• On allocation failure, returns the old brk_current (Linux convention: failure = unchanged break).

Initial state: brk_base and brk_current are set to the page-aligned end of the highest PT_LOAD ELF segment when the process is spawned.

Lock ordering: Process table lock is acquired/released to read state, then memory lock for allocation, then process table lock again to write the update.

Source: osl/src/syscalls/mem.rs — sys_brk

Future Work

• Free physical frames and unmap pages when the break is decreased.
• Guard against growing the break into other mapped regions (stack, mmap area).