From 62c8421e87b33b2e94f8a7842e1dd9aa1a286ffc Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 7 Jul 2016 11:28:17 -0400
Subject: [PATCH] Reduce stack space consumption in tzload().
While syncing our timezone code with IANA's updates in commit 1c1a7cbd6,
I'd chosen not to adopt the code they conditionally compile under #ifdef
ALL_STATE. The main thing that that drives is that the space for gmtime
and localtime timezone definitions isn't statically allocated, but is
malloc'd on first use. I reasoned we didn't need that logic: we don't have
localtime() at all, and we always initialize TimeZone to GMT so we always
need that one. But there is one other thing ALL_STATE does, which is to
make tzload() malloc its transient workspace instead of just declaring it
as a local variable. It turns out that that local variable occupies 78K.
Even worse is that, at least for common US timezone settings, there's a
recursive call to parse the "posixrules" zone name, making peak stack
consumption to select a time zone upwards of 150K. That's an uncomfortably
large fraction of our STACK_DEPTH_SLOP safety margin, and could result in
outright crashes if we try to reduce STACK_DEPTH_SLOP as has been discussed
recently. Furthermore, this means that the postmaster's peak stack
consumption is several times that of a backend running typical queries
(since, except on Windows, backends inherit the timezone GUC values and
don't ever run this code themselves unless you do SET TIMEZONE). That's
completely backwards from a safety perspective.
Hence, adopt the ALL_STATE rather than non-ALL_STATE variant of tzload(),
while not changing the other code aspects that symbol controls. The
risk of an ENOMEM error from malloc() seems less than that of a SIGSEGV
from stack overrun.
This should probably get back-patched along with 1c1a7cbd6 and followon
fixes, whenever we decide we have enough confidence in the updates to do
that.
---
src/timezone/localtime.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/src/timezone/localtime.c b/src/timezone/localtime.c
index 985ad56da20..a14215d6bde 100644
--- a/src/timezone/localtime.c
+++ b/src/timezone/localtime.c
@@ -536,9 +536,17 @@ tzloadbody(char const * name, char *canonname, struct state * sp, bool doextend,
int
tzload(const char *name, char *canonname, struct state * sp, bool doextend)
{
- union local_storage ls;
+ union local_storage *lsp = malloc(sizeof *lsp);
- return tzloadbody(name, canonname, sp, doextend, &ls);
+ if (!lsp)
+ return errno;
+ else
+ {
+ int err = tzloadbody(name, canonname, sp, doextend, lsp);
+
+ free(lsp);
+ return err;
+ }
}
static bool
--
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