On Mon, Dec 28, 2020 at 5:24 PM Waldek Kozaczuk <jwkozac...@gmail.com>
wrote:
> How did you discover this bug?
>
I am trying to run Lua 5.4, and because of misconfiguration, I caused it to
read a file with an empty name (!)
and this resulted in read errors, and fread() just went into an endless
loop and I had to debug why....
Then I found the incorrect logic in fread(), and "git blame" told me the
logic is correct, if you change __stdio_read() accordingly - and it turns
out that's what they did in Musl.
I was too lazy to write a unit test that reproduces the bug. But I should!
It should be very easy (a simple fread() from a directory should result in
an endless loop) I will do it later.
> Sent from my iPhone
>
> > On Dec 28, 2020, at 08:54, Nadav Har'El <n...@scylladb.com> wrote:
> >
> > We have our own copy of __stdio_read.c because it was too complicated
> > to take Musl's as-is because of locking differences. However, there was
> > one recent change to that file we forgot to take:
> >
> > In commit f92804188 in Musl's repository, Rich Felker (in Feb. 2018),
> > changed fread() to assume that f->read() returns 0 on error, instead
> > of -1. The same patch fixes __stdio_read() to return 0, and we need
> > to do this as well.
> >
> > Without this patch, when the new Musl's fread() encounters an error
> (e.g.,
> > when you try to read from a directory), it will go into an infinite
> > loop - each iteration increases the remaining bytes (by 1) instead of
> > decreasing it by the number of read bytes as normal.
> >
> > Signed-off-by: Nadav Har'El <n...@scylladb.com>
> > ---
> > libc/stdio/__stdio_read.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/libc/stdio/__stdio_read.c b/libc/stdio/__stdio_read.c
> > index 8fec9c35..30d7aac5 100644
> > --- a/libc/stdio/__stdio_read.c
> > +++ b/libc/stdio/__stdio_read.c
> > @@ -30,7 +30,7 @@ size_t __stdio_read(FILE *f, unsigned char *buf,
> size_t len)
> > if (cnt <= 0) {
> > f->flags |= F_EOF ^ ((F_ERR^F_EOF) & cnt);
> > f->rpos = f->rend = 0;
> > - return cnt;
> > + return 0;
> > }
> > if (cnt <= len) return cnt;
> > cnt -= len;
> > --
> > 2.29.2
> >
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>
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