Re: [PATCH v8 2/6] util: Introduce read_file() and read_file_integer() function

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Nov 19, 2025 at 05:04:31PM +0800, Yumei Huang wrote:
> On Tue, Nov 18, 2025 at 11:36 AM David Gibson
> <david@gibson.dropbear.id.au> wrote:
> >
> > On Tue, Nov 18, 2025 at 01:19:38AM +0100, Stefano Brivio wrote:
> > > On Mon, 10 Nov 2025 17:31:33 +0800
> > > Yumei Huang <yuhuang@redhat.com> wrote:
> > >
> > > > Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> > > > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> > > > ---
> > > >  util.c | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >  util.h |  2 ++
> > > >  2 files changed, 88 insertions(+)
> > > >
> > > > diff --git a/util.c b/util.c
> > > > index 44c21a3..c4c849c 100644
> > > > --- a/util.c
> > > > +++ b/util.c
> > > > @@ -590,6 +590,92 @@ int write_file(const char *path, const char *buf)
> > > >     return len == 0 ? 0 : -1;
> > > >  }
> > > >
> > > > +/**
> > > > + * read_file() - Read contents of file into a NULL-terminated buffer
> > > > + * @path:  Path to file to read
> > > > + * @buf:   Buffer to store file contents
> > > > + * @buf_size:      Size of buffer
> > > > + *
> > > > + * Return: number of bytes read on success, -1 on error, -ENOBUFS on truncation
> > > > + */
> > > > +ssize_t read_file(const char *path, char *buf, size_t buf_size)
> > > > +{
> > > > +   int fd = open(path, O_RDONLY | O_CLOEXEC);
> > > > +   size_t total_read = 0;
> > > > +   ssize_t rc;
> > > > +
> > > > +   if (fd < 0) {
> > > > +           warn_perror("Could not open %s", path);
> > >
> > > While testing something unrelated I realised that this looks like a
> > > serious failure, but it's perfectly normal on (even slightly) older
> > > kernels:
> > >
> > > ---
> > > $ git describe --contains ccce324dabfe # tcp: make the first N SYN RTO backoffs linear
> > > v6.5-rc1~163^2~299
> > > $ git describe --contains 1280c26228bd # tcp: add tcp_rto_max_ms sysctl
> > > v6.15-rc1~160^2~352^2
> > > ---
> > >
> > > On a 6.1 kernel, for example:
> > >
> > > ---
> > > $ ./pasta -d --config-net
> > > [...]
> > > 0.0258: Could not open /proc/sys/net/ipv4/tcp_syn_linear_timeouts: No such file or directory
> > > 0.0258: Could not open /proc/sys/net/ipv4/tcp_rto_max_ms: No such file or directory
> > > 0.0258: Read sysctl values syn_retries: 6, syn_linear_timeouts: 4, rto_max: 120
> > > ---
> > >
> > > and actually the third message isn't accurate, because we didn't read
> > > those values, we are just using them.
> >
> > Right.  In fact I'd say here the fact they came from sysctl is less
> > important here than what we're using them for.  So maybe
> >         Using TCP RTO parameters, syn_retries: 6, ...
> 
> Got it.
> >
> > >
> > > Worse, yet:
> > >
> > > ---
> > > $ ./pasta
> > > Could not open /proc/sys/net/ipv4/tcp_syn_linear_timeouts: No such file or directory
> > > Could not open /proc/sys/net/ipv4/tcp_rto_max_ms: No such file or directory
> > > ---
> > >
> > > so this would be logged *with default options* by Podman, rootlesskit /
> > > Docker, via libvirt, and by other users too, meaning we would get
> > > submerged by reports about this "error" if we release it like this (6.15
> > > is fairly recent).
> > >
> > > So maybe we could turn this (and the messages below) to debug() /
> > > debug_perror(), and then:
> >
> > I think we should remove the error message from read_file() entirely,
> > just returning an error code.  Essentially it is too low level to know
> > the severity and meaning of a failure, so reporting the problem to the
> > user is better left to the valler.
> 
> Then we will have a few more values to return, like errno for open or
> read file fails, -ENOBUFS, and another error code for when buf size is
> 0.  I'd say it's more complicated than write_fiel(). Maybe we could
> update with debug()/debug_perror() first as Stefano suggested, and
> then fix them together later?

Right, returning full error details is somewhere between awkward and
impossible in C.  I think returning -errno for the first error to
occur is probably good enough, though.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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