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amcheck: Tighten up validation of redirect line pointers. 

Commit bbc1376b39 added a new lp_valid[]
array which records whether or not a line pointer was thought to be
valid, but entries could sometimes get set to true in cases where that
wasn't actually safe. Fix that.

Suppose A is a redirect line pointer and B is the other line pointer
to which it points.  The old code could mishandle this situation in a
couple of different ways.  First, if B was unused, we'd complain about
corruption but still set lp_valid[A] = true, causing later code
to try to access the B as if it were pointing to a tuple. Second,
if B was dead, we wouldn't complain about corruption at all, which is
an oversight, and would also set lp_valid[A] = true, which would
again confuse later code. Fix all that.

In the case where B is a redirect, the old code was correct, but
refactor things a bit anyway so that all of these cases are handled
more symmetrically. Also add an Assert() and some comments.

Andres Freund and Robert Haas

Discussion: http://postgr.es/m/20230323172607.y3lejpntjnuis5vv%40awork3.anarazel.de
This commit is contained in:
Robert Haas 2023-03-27 13:27:06 -04:00
parent 5a91c79755
commit 80d5e3a615
1 changed files with 31 additions and 9 deletions
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40
contrib/amcheck/verify_heapam.c
View File

@ -481,11 +481,35 @@ verify_heapam(PG_FUNCTION_ARGS)
(unsigned) maxoff));
continue;
}
/*
* Since we've checked that this redirect points to a line
* pointer between FirstOffsetNumber and maxoff, it should
* now be safe to fetch the referenced line pointer. We expect
* it to be LP_NORMAL; if not, that's corruption.
*/
rditem = PageGetItemId(ctx.page, rdoffnum);
if (!ItemIdIsUsed(rditem))
{
report_corruption(&ctx,
psprintf("line pointer redirection to unused item at offset %u",
psprintf("redirected line pointer points to an unused item at offset %u",
(unsigned) rdoffnum));
continue;
}
else if (ItemIdIsDead(rditem))
{
report_corruption(&ctx,
psprintf("redirected line pointer points to a dead item at offset %u",
(unsigned) rdoffnum));
continue;
}
else if (ItemIdIsRedirected(rditem))
{
report_corruption(&ctx,
psprintf("redirected line pointer points to another redirected line pointer at offset %u",
(unsigned) rdoffnum));
continue;
}
/*
* Record the fact that this line pointer has passed basic
@ -584,14 +608,12 @@ verify_heapam(PG_FUNCTION_ARGS)
/* Handle the cases where the current line pointer is a redirect. */
if (ItemIdIsRedirected(curr_lp))
{
/* Can't redirect to another redirect. */
if (ItemIdIsRedirected(next_lp))
{
report_corruption(&ctx,
psprintf("redirected line pointer points to another redirected line pointer at offset %u",
(unsigned) nextoffnum));
continue;
}
/*
* We should not have set successor[ctx.offnum] to a value
* other than InvalidOffsetNumber unless that line pointer
* is LP_NORMAL.
*/
Assert(ItemIdIsNormal(next_lp));
/* Can only redirect to a HOT tuple. */
next_htup = (HeapTupleHeader) PageGetItem(ctx.page, next_lp);