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[linux] Avoid starting currticks() from zero every time

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iPXE uses currticks() (along with the MAC address(es) of any network
devices) to seed the (non-cryptographic) random number generator.  The
current implementation of linux_currticks() ensures that the first
call to currticks() will always return zero; this results in identical
random number sequences on each run of iPXE on a given machine.  This
can cause odd-looking behaviour due to e.g. the reuse of local TCP
port numbers.

Fix by effectively rounding down the start time recorded by
linux_currticks() to the nearest whole second; this makes it unlikely
that consecutive runs of iPXE will use the exact same RNG sequence.

(Note that none of this affects the cryptographic RNG, which uses
/dev/random as a source of entropy.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2014-03-07 16:40:36 +00:00
parent 859664ea2a
commit 08f9170ba4
1 changed files with 7 additions and 1 deletions
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8
src/interface/linux/linux_timer.c
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@@ -55,6 +55,12 @@ static unsigned long linux_ticks_per_sec(void)
* linux doesn't provide an easy access to jiffies so implement it by measuring * linux doesn't provide an easy access to jiffies so implement it by measuring
* the time since the first call to this function. * the time since the first call to this function.
* *
* Since this function is used to seed the (non-cryptographic) random
* number generator, we round the start time down to the nearest whole
* second. This minimises the chances of generating identical RNG
* sequences (and hence identical TCP port numbers, etc) on
* consecutive invocations of iPXE.
*
* @ret ticks Current time, in ticks * @ret ticks Current time, in ticks
*/ */
static unsigned long linux_currticks(void) static unsigned long linux_currticks(void)
@@ -71,7 +77,7 @@ static unsigned long linux_currticks(void)
linux_gettimeofday(&now, NULL); linux_gettimeofday(&now, NULL);
unsigned long ticks = (now.tv_sec - start.tv_sec) * linux_ticks_per_sec(); unsigned long ticks = (now.tv_sec - start.tv_sec) * linux_ticks_per_sec();
ticks += (now.tv_usec - start.tv_usec) / (long)(1000000 / linux_ticks_per_sec()); ticks += now.tv_usec / (long)(1000000 / linux_ticks_per_sec());
return ticks; return ticks;
} }