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Date: Wed, 15 Dec 2021 17:30:23 +0800 (CST)
From: tugouxp <13824125580@....com>
To: musl@...ts.openwall.com
Subject: Did the musl libc never decrease the brk pointer of Kernel? just
increase ?
Hi guys:
i found i intresting things when i fix a memory leak issue on may platform which based on musl c library. the issue has been fixed but a puzzle leave it to me.
in the file of malloc/mallocng/malloc.c, a function called "alloc_meta" says that as belows,so you can see the brk pointer of brk system call parmeter never decrease the brk, is not it ? did gilibc also does like this way?
why design like this, thank you !
struct meta *alloc_meta(void)
{
struct meta *m;
unsigned char *p;
if (!ctx.init_done) {
#ifndef PAGESIZE
ctx.pagesize = get_page_size();
#endif
ctx.secret = get_random_secret();
ctx.init_done = 1;
}
size_t pagesize = PGSZ;
if (pagesize < 4096) pagesize = 4096;
if ((m = dequeue_head(&ctx.free_meta_head))) return m;
if (!ctx.avail_meta_count) {
int need_unprotect = 1;
if (!ctx.avail_meta_area_count && ctx.brk!=-1) {
uintptr_t new = ctx.brk + pagesize;
int need_guard = 0;
if (!ctx.brk) {
need_guard = 1;
ctx.brk = brk(0);
// some ancient kernels returned _ebss
// instead of next page as initial brk.
ctx.brk += -ctx.brk & (pagesize-1);
new = ctx.brk + 2*pagesize;
}
if (brk(new) != new) {
ctx.brk = -1;
} else {
if (need_guard) mmap((void *)ctx.brk, pagesize,
PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
ctx.brk = new;
ctx.avail_meta_areas = (void *)(new - pagesize);
ctx.avail_meta_area_count = pagesize>>12;
need_unprotect = 0;
}
}
if (!ctx.avail_meta_area_count) {
size_t n = 2UL << ctx.meta_alloc_shift;
p = mmap(0, n*pagesize, PROT_NONE,
MAP_PRIVATE|MAP_ANON, -1, 0);
if (p==MAP_FAILED) return 0;
ctx.avail_meta_areas = p + pagesize;
ctx.avail_meta_area_count = (n-1)*(pagesize>>12);
ctx.meta_alloc_shift++;
/\<brk\> 63,15-43 10%
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