184 lines
6.1 KiB
Diff
184 lines
6.1 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Axtens <dja@axtens.net>
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Date: Thu, 25 Nov 2021 02:22:46 +1100
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Subject: [PATCH] mm: Clarify grub_real_malloc()
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When iterating through the singly linked list of free blocks,
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grub_real_malloc() uses p and q for the current and previous blocks
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respectively. This isn't super clear, so swap to using prev and cur.
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This makes another quirk more obvious. The comment at the top of
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grub_real_malloc() might lead you to believe that the function will
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allocate from *first if there is space in that block.
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It actually doesn't do that, and it can't do that with the current
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data structures. If we used up all of *first, we would need to change
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the ->next of the previous block to point to *first->next, but we
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can't do that because it's a singly linked list and we don't have
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access to *first's previous block.
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What grub_real_malloc() actually does is set *first to the initial
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previous block, and *first->next is the block we try to allocate
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from. That allows us to keep all the data structures consistent.
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Document that.
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Signed-off-by: Daniel Axtens <dja@axtens.net>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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(cherry picked from commit 246ad6a44c281bb13486ddea0a26bb661db73106)
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---
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grub-core/kern/mm.c | 76 +++++++++++++++++++++++++++++------------------------
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1 file changed, 41 insertions(+), 35 deletions(-)
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diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
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index d8c8377578..fb20e93acf 100644
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--- a/grub-core/kern/mm.c
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+++ b/grub-core/kern/mm.c
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@@ -178,13 +178,20 @@ grub_mm_init_region (void *addr, grub_size_t size)
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}
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/* Allocate the number of units N with the alignment ALIGN from the ring
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- buffer starting from *FIRST. ALIGN must be a power of two. Both N and
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- ALIGN are in units of GRUB_MM_ALIGN. Return a non-NULL if successful,
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- otherwise return NULL. */
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+ * buffer given in *FIRST. ALIGN must be a power of two. Both N and
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+ * ALIGN are in units of GRUB_MM_ALIGN. Return a non-NULL if successful,
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+ * otherwise return NULL.
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+ *
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+ * Note: because in certain circumstances we need to adjust the ->next
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+ * pointer of the previous block, we iterate over the singly linked
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+ * list with the pair (prev, cur). *FIRST is our initial previous, and
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+ * *FIRST->next is our initial current pointer. So we will actually
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+ * allocate from *FIRST->next first and *FIRST itself last.
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+ */
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static void *
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grub_real_malloc (grub_mm_header_t *first, grub_size_t n, grub_size_t align)
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{
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- grub_mm_header_t p, q;
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+ grub_mm_header_t cur, prev;
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/* When everything is allocated side effect is that *first will have alloc
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magic marked, meaning that there is no room in this region. */
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@@ -192,24 +199,24 @@ grub_real_malloc (grub_mm_header_t *first, grub_size_t n, grub_size_t align)
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return 0;
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/* Try to search free slot for allocation in this memory region. */
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- for (q = *first, p = q->next; ; q = p, p = p->next)
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+ for (prev = *first, cur = prev->next; ; prev = cur, cur = cur->next)
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{
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grub_off_t extra;
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- extra = ((grub_addr_t) (p + 1) >> GRUB_MM_ALIGN_LOG2) & (align - 1);
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+ extra = ((grub_addr_t) (cur + 1) >> GRUB_MM_ALIGN_LOG2) & (align - 1);
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if (extra)
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extra = align - extra;
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- if (! p)
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+ if (! cur)
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grub_fatal ("null in the ring");
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- if (p->magic != GRUB_MM_FREE_MAGIC)
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- grub_fatal ("free magic is broken at %p: 0x%x", p, p->magic);
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+ if (cur->magic != GRUB_MM_FREE_MAGIC)
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+ grub_fatal ("free magic is broken at %p: 0x%x", cur, cur->magic);
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- if (p->size >= n + extra)
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+ if (cur->size >= n + extra)
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{
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- extra += (p->size - extra - n) & (~(align - 1));
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- if (extra == 0 && p->size == n)
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+ extra += (cur->size - extra - n) & (~(align - 1));
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+ if (extra == 0 && cur->size == n)
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{
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/* There is no special alignment requirement and memory block
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is complete match.
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@@ -222,9 +229,9 @@ grub_real_malloc (grub_mm_header_t *first, grub_size_t n, grub_size_t align)
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| alloc, size=n | |
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+---------------+ v
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*/
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- q->next = p->next;
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+ prev->next = cur->next;
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}
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- else if (align == 1 || p->size == n + extra)
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+ else if (align == 1 || cur->size == n + extra)
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{
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/* There might be alignment requirement, when taking it into
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account memory block fits in.
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@@ -241,23 +248,22 @@ grub_real_malloc (grub_mm_header_t *first, grub_size_t n, grub_size_t align)
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| alloc, size=n | |
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+---------------+ v
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*/
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-
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- p->size -= n;
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- p += p->size;
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+ cur->size -= n;
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+ cur += cur->size;
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}
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else if (extra == 0)
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{
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grub_mm_header_t r;
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- r = p + extra + n;
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+ r = cur + extra + n;
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r->magic = GRUB_MM_FREE_MAGIC;
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- r->size = p->size - extra - n;
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- r->next = p->next;
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- q->next = r;
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+ r->size = cur->size - extra - n;
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+ r->next = cur->next;
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+ prev->next = r;
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- if (q == p)
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+ if (prev == cur)
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{
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- q = r;
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+ prev = r;
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r->next = r;
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}
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}
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@@ -284,32 +290,32 @@ grub_real_malloc (grub_mm_header_t *first, grub_size_t n, grub_size_t align)
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*/
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grub_mm_header_t r;
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- r = p + extra + n;
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+ r = cur + extra + n;
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r->magic = GRUB_MM_FREE_MAGIC;
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- r->size = p->size - extra - n;
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- r->next = p;
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+ r->size = cur->size - extra - n;
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+ r->next = cur;
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- p->size = extra;
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- q->next = r;
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- p += extra;
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+ cur->size = extra;
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+ prev->next = r;
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+ cur += extra;
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}
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- p->magic = GRUB_MM_ALLOC_MAGIC;
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- p->size = n;
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+ cur->magic = GRUB_MM_ALLOC_MAGIC;
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+ cur->size = n;
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/* Mark find as a start marker for next allocation to fasten it.
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This will have side effect of fragmenting memory as small
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pieces before this will be un-used. */
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/* So do it only for chunks under 64K. */
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if (n < (0x8000 >> GRUB_MM_ALIGN_LOG2)
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- || *first == p)
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- *first = q;
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+ || *first == cur)
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+ *first = prev;
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- return p + 1;
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+ return cur + 1;
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}
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/* Search was completed without result. */
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- if (p == *first)
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+ if (cur == *first)
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break;
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}
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