| Message ID | 20211018122144.25131-1-andrew.shadura@collabora.co.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Delegated to: | Jiri Kosina |
| Headers | show |
| Series | [v2,1/2] HID: u2fzero: explicitly check for errors | expand |
On Mon, Oct 18, 2021 at 02:21:43PM +0200, Andrej Shadura wrote: > The previous commit fixed handling of incomplete packets but broke error > handling: offsetof returns an unsigned value (size_t), but when compared > against the signed return value, the return value is interpreted as if > it were unsigned, so negative return values are never less than the > offset. > > Fixes: 22d65765f211 ("HID: u2fzero: ignore incomplete packets without data") > Fixes: 42337b9d4d95 ("HID: add driver for U2F Zero built-in LED and RNG") > Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk> > --- > drivers/hid/hid-u2fzero.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/hid/hid-u2fzero.c b/drivers/hid/hid-u2fzero.c > index d70cd3d7f583..5145d758bea0 100644 > --- a/drivers/hid/hid-u2fzero.c > +++ b/drivers/hid/hid-u2fzero.c > @@ -200,7 +200,7 @@ static int u2fzero_rng_read(struct hwrng *rng, void *data, > ret = u2fzero_recv(dev, &req, &resp); > > /* ignore errors or packets without data */ > - if (ret < offsetof(struct u2f_hid_msg, init.data)) > + if (ret < 0 || ret < offsetof(struct u2f_hid_msg, init.data)) Although the patch description does a good job of explaining what's happening, someone merely reading the code will most likely not understand. One alternative is to add a comment. Another is simply to force a signed integer comparison: if (ret < (ssize_t) offsetof(... Alan Stern
On 18/10/2021 16:15, Alan Stern wrote: > On Mon, Oct 18, 2021 at 02:21:43PM +0200, Andrej Shadura wrote: >> The previous commit fixed handling of incomplete packets but broke error >> handling: offsetof returns an unsigned value (size_t), but when compared >> against the signed return value, the return value is interpreted as if >> it were unsigned, so negative return values are never less than the >> offset. >> >> Fixes: 22d65765f211 ("HID: u2fzero: ignore incomplete packets without data") >> Fixes: 42337b9d4d95 ("HID: add driver for U2F Zero built-in LED and RNG") >> Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk> >> --- >> drivers/hid/hid-u2fzero.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/drivers/hid/hid-u2fzero.c b/drivers/hid/hid-u2fzero.c >> index d70cd3d7f583..5145d758bea0 100644 >> --- a/drivers/hid/hid-u2fzero.c >> +++ b/drivers/hid/hid-u2fzero.c >> @@ -200,7 +200,7 @@ static int u2fzero_rng_read(struct hwrng *rng, void *data, >> ret = u2fzero_recv(dev, &req, &resp); >> >> /* ignore errors or packets without data */ >> - if (ret < offsetof(struct u2f_hid_msg, init.data)) >> + if (ret < 0 || ret < offsetof(struct u2f_hid_msg, init.data)) > > Although the patch description does a good job of explaining what's > happening, someone merely reading the code will most likely not > understand. > > One alternative is to add a comment. Another is simply to force a > signed integer comparison: > > if (ret < (ssize_t) offsetof(... I have considered that, but I thought that is actually less readable than having two conditions. I’m curious that you say "ignore errors or packets without data" is not clear enough — how would you reword that without inflating it too much?
On Mon, Oct 18, 2021 at 04:17:57PM +0200, Andrej Shadura wrote: > On 18/10/2021 16:15, Alan Stern wrote: > > On Mon, Oct 18, 2021 at 02:21:43PM +0200, Andrej Shadura wrote: > > > The previous commit fixed handling of incomplete packets but broke error > > > handling: offsetof returns an unsigned value (size_t), but when compared > > > against the signed return value, the return value is interpreted as if > > > it were unsigned, so negative return values are never less than the > > > offset. > > > > > > Fixes: 22d65765f211 ("HID: u2fzero: ignore incomplete packets without data") > > > Fixes: 42337b9d4d95 ("HID: add driver for U2F Zero built-in LED and RNG") > > > Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk> > > > --- > > > drivers/hid/hid-u2fzero.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/drivers/hid/hid-u2fzero.c b/drivers/hid/hid-u2fzero.c > > > index d70cd3d7f583..5145d758bea0 100644 > > > --- a/drivers/hid/hid-u2fzero.c > > > +++ b/drivers/hid/hid-u2fzero.c > > > @@ -200,7 +200,7 @@ static int u2fzero_rng_read(struct hwrng *rng, void *data, > > > ret = u2fzero_recv(dev, &req, &resp); > > > /* ignore errors or packets without data */ > > > - if (ret < offsetof(struct u2f_hid_msg, init.data)) > > > + if (ret < 0 || ret < offsetof(struct u2f_hid_msg, init.data)) > > > > Although the patch description does a good job of explaining what's > > happening, someone merely reading the code will most likely not > > understand. > > > > One alternative is to add a comment. Another is simply to force a > > signed integer comparison: > > > > if (ret < (ssize_t) offsetof(... > > I have considered that, but I thought that is actually less readable than > having two conditions. I’m curious that you say "ignore errors or packets > without data" is not clear enough — how would you reword that without > inflating it too much? You misunderstand. The existing comment is clear enough. But the code itself is misleading: if (ret < 0 || ret < offsetof(... looks redundant. Someone reading it for the first time will automatically think: "If ret < 0 then certainly it is < the offset of some internal field. So why perform two comparisons when one is enough?" To help such a reader understand what is happening, you could add a comment like: /* * offsetof returns an unsigned value, so the comparison with * ret uses unsigned arithmetic and won't detect a negative * error value. We need a separate test for errors. */ If you think a comment like this is preferable to a typecast, fine. Another alternative is: ret -= offsetof(...); if (ret < 0) return 0; which may look more complicated but allows you to simplify the max3 computation in the next line. Alan Stern
diff --git a/drivers/hid/hid-u2fzero.c b/drivers/hid/hid-u2fzero.c index d70cd3d7f583..5145d758bea0 100644 --- a/drivers/hid/hid-u2fzero.c +++ b/drivers/hid/hid-u2fzero.c @@ -200,7 +200,7 @@ static int u2fzero_rng_read(struct hwrng *rng, void *data, ret = u2fzero_recv(dev, &req, &resp); /* ignore errors or packets without data */ - if (ret < offsetof(struct u2f_hid_msg, init.data)) + if (ret < 0 || ret < offsetof(struct u2f_hid_msg, init.data)) return 0; /* only take the minimum amount of data it is safe to take */
The previous commit fixed handling of incomplete packets but broke error handling: offsetof returns an unsigned value (size_t), but when compared against the signed return value, the return value is interpreted as if it were unsigned, so negative return values are never less than the offset. Fixes: 22d65765f211 ("HID: u2fzero: ignore incomplete packets without data") Fixes: 42337b9d4d95 ("HID: add driver for U2F Zero built-in LED and RNG") Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk> --- drivers/hid/hid-u2fzero.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)