On Fri, Sep 16, 2022 at 02:41:30PM +0100, Josh Triplett wrote: > Currently, execve allocates an mm and parses argv and envp before > checking if the path exists. However, the common case of a $PATH search > may have several failed calls to exec before a single success. Do a > filename lookup for the purposes of returning ENOENT before doing more > expensive operations. > > This does not create a TOCTTOU race, because this can only happen if the > file didn't exist at some point during the exec call, and that point is > permitted to be when we did our lookup. > > To measure performance, I ran 2000 fork and execvpe calls with a > seven-element PATH in which the file was found in the seventh directory > (representative of the common case as /usr/bin is the seventh directory > on my $PATH), as well as 2000 fork and execve calls with an absolute > path to an existing binary. I recorded the minimum time for each, to > eliminate noise from context switches and similar. > > Without fast-path: > fork/execvpe: 49876ns > fork/execve: 32773ns > > With fast-path: > fork/execvpe: 36890ns > fork/execve: 32069ns > > The cost of the additional lookup seems to be in the noise for a > successful exec, but it provides a 26% improvement for the path search > case by speeding up the six failed execs. > > Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> *thread necromancy* I'll snag this patch after -rc1 is out. Based on the research we both did in the rest of this thread, this original patch is a clear win. Let's get it into linux-next and see if anything else falls out of it. I did, however, scratch my head over the 0-day report: https://lore.kernel.org/lkml/202209221401.90061e56-yujie.liu@xxxxxxxxx/ But I can't see why this patch could trigger those problems... Thanks! -Kees -- Kees Cook
