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Make checksum_impl.h safe to compile with -fstrict-aliasing.

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In general, Postgres requires -fno-strict-aliasing with compilers that
implement C99 strict aliasing rules.  There's little hope of getting
rid of that overall.  But it seems like it would be a good idea if
storage/checksum_impl.h in particular didn't depend on it, because
that header is explicitly intended to be included by external programs.
We don't have a lot of control over the compiler switches that an
external program might use, as shown by Michael Banck's report of
failure in a privately-modified version of pg_verify_checksums.

Hence, switch to using a union in place of willy-nilly pointer casting
inside this file.  I think this makes the code a bit more readable
anyway.

checksum_impl.h hasn't changed since it was introduced in 9.3,
so back-patch all the way.

Discussion: https://postgr.es/m/1535618100.1286.3.camel@credativ.de
This commit is contained in:
Tom Lane 2018-08-31 12:26:20 -04:00
parent 7cfdc77023
commit 8c62d9d16f
1 changed files with 23 additions and 15 deletions
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38
src/include/storage/checksum_impl.h
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@ -107,6 +107,13 @@
/* prime multiplier of FNV-1a hash */
#define FNV_PRIME 16777619
/* Use a union so that this code is valid under strict aliasing */
typedef union
{
PageHeaderData phdr;
uint32 data[BLCKSZ / (sizeof(uint32) * N_SUMS)][N_SUMS];
} PGChecksummablePage;
/*
* Base offsets to initialize each of the parallel FNV hashes into a
* different initial state.
@ -132,28 +139,27 @@ do { \
} while (0)
/*
* Block checksum algorithm. The data argument must be aligned on a 4-byte
* boundary.
* Block checksum algorithm. The page must be adequately aligned
* (at least on 4-byte boundary).
*/
static uint32
pg_checksum_block(char *data, uint32 size)
pg_checksum_block(const PGChecksummablePage *page)
{
uint32 sums[N_SUMS];
uint32 (*dataArr)[N_SUMS] = (uint32 (*)[N_SUMS]) data;
uint32 result = 0;
uint32 i,
j;
/* ensure that the size is compatible with the algorithm */
Assert((size % (sizeof(uint32) * N_SUMS)) == 0);
Assert(sizeof(PGChecksummablePage) == BLCKSZ);
/* initialize partial checksums to their corresponding offsets */
memcpy(sums, checksumBaseOffsets, sizeof(checksumBaseOffsets));
/* main checksum calculation */
for (i = 0; i < size / sizeof(uint32) / N_SUMS; i++)
for (i = 0; i < (uint32) (BLCKSZ / (sizeof(uint32) * N_SUMS)); i++)
for (j = 0; j < N_SUMS; j++)
CHECKSUM_COMP(sums[j], dataArr[i][j]);
CHECKSUM_COMP(sums[j], page->data[i][j]);
/* finally add in two rounds of zeroes for additional mixing */
for (i = 0; i < 2; i++)
@ -168,8 +174,10 @@ pg_checksum_block(char *data, uint32 size)
}
/*
* Compute the checksum for a Postgres page. The page must be aligned on a
* 4-byte boundary.
* Compute the checksum for a Postgres page.
*
* The page must be adequately aligned (at least on a 4-byte boundary).
* Beware also that the checksum field of the page is transiently zeroed.
*
* The checksum includes the block number (to detect the case where a page is
* somehow moved to a different location), the page header (excluding the
@ -178,12 +186,12 @@ pg_checksum_block(char *data, uint32 size)
uint16
pg_checksum_page(char *page, BlockNumber blkno)
{
PageHeader phdr = (PageHeader) page;
PGChecksummablePage *cpage = (PGChecksummablePage *) page;
uint16 save_checksum;
uint32 checksum;
/* We only calculate the checksum for properly-initialized pages */
Assert(!PageIsNew(page));
Assert(!PageIsNew(&cpage->phdr));
/*
* Save pd_checksum and temporarily set it to zero, so that the checksum
@ -191,10 +199,10 @@ pg_checksum_page(char *page, BlockNumber blkno)
* Restore it after, because actually updating the checksum is NOT part of
* the API of this function.
*/
save_checksum = phdr->pd_checksum;
phdr->pd_checksum = 0;
checksum = pg_checksum_block(page, BLCKSZ);
phdr->pd_checksum = save_checksum;
save_checksum = cpage->phdr.pd_checksum;
cpage->phdr.pd_checksum = 0;
checksum = pg_checksum_block(cpage);
cpage->phdr.pd_checksum = save_checksum;
/* Mix in the block number to detect transposed pages */
checksum ^= blkno;