Fork of alistair23 Linux kernel for reMarkable from https://github.com/alistair23/linux
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Patch series "Fortify strscpy()", v7.
This patch implements a fortified version of strscpy() enabled by setting
CONFIG_FORTIFY_SOURCE=y. The new version ensures the following before
calling vanilla strscpy():
1. There is no read overflow because either size is smaller than src
length or we shrink size to src length by calling fortified strnlen().
2. There is no write overflow because we either failed during
compilation or at runtime by checking that size is smaller than dest
size. Note that, if src and dst size cannot be got, the patch defaults
to call vanilla strscpy().
The patches adds the following:
1. Implement the fortified version of strscpy().
2. Add a new LKDTM test to ensures the fortified version still returns
the same value as the vanilla one while panic'ing when there is a write
overflow.
3. Correct some typos in LKDTM related file.
I based my modifications on top of two patches from Daniel Axtens which
modify calls to __builtin_object_size, in fortified string functions, to
ensure the true size of char * are returned and not the surrounding
structure size.
About performance, I measured the slow down of fortified strscpy(), using
the vanilla one as baseline. The hardware I used is an Intel i3 2130 CPU
clocked at 3.4 GHz. I ran "Linux 5.10.0-rc4+ SMP PREEMPT" inside qemu
3.10 with 4 CPU cores. The following code, called through LKDTM, was used
as a benchmark:
#define TIMES 10000
char *src;
char dst[7];
int i;
ktime_t begin;
src = kstrdup("foobar", GFP_KERNEL);
if (src == NULL)
return;
begin = ktime_get();
for (i = 0; i < TIMES; i++)
strscpy(dst, src, strlen(src));
pr_info("%d fortified strscpy() tooks %lld", TIMES, ktime_get() - begin);
begin = ktime_get();
for (i = 0; i < TIMES; i++)
__real_strscpy(dst, src, strlen(src));
pr_info("%d vanilla strscpy() tooks %lld", TIMES, ktime_get() - begin);
kfree(src);
I called the above code 30 times to compute stats for each version (in ns,
round to int):
| version | mean | std | median | 95th |
| --------- | ------- | ------ | ------- | ------- |
| fortified | 245_069 | 54_657 | 216_230 | 331_122 |
| vanilla | 172_501 | 70_281 | 143_539 | 219_553 |
On average, fortified strscpy() is approximately 1.42 times slower than
vanilla strscpy(). For the 95th percentile, the fortified version is
about 1.50 times slower.
So, clearly the stats are not in favor of fortified strscpy(). But, the
fortified version loops the string twice (one in strnlen() and another in
vanilla strscpy()) while the vanilla one only loops once. This can
explain why fortified strscpy() is slower than the vanilla one.
This patch (of 5):
When the fortify feature was first introduced in commit
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README
Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
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the problems which may result by upgrading your kernel.