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plugin/hosts: don't drop over-long fields silently (#8551)

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#8496 fixed a silent truncation here and named the invariant in its own commit
message: entries were dropped and the hosts file simply looked shorter than it
is, with nothing in the log.

#8516 replaced that mechanism with a streaming parser bounded by maxFieldSize.
The error log #8496 added is still in parse(), but it can no longer report a
dropped entry: bufio.ErrBufferFull is consumed by the read loop, so only a real
I/O error reaches it. A field over maxFieldSize is discarded in lineParser with
no log at all, and when that field is the address the whole line goes with it.

Report both cases, once per dropped field, with the line number and the source
the entries came from.

Signed-off-by: Baltasar Blanco <baltasarblanco.dev@gmail.com>
This commit is contained in:
Baltasar Blanco
2026-09-15 23:24:23 -03:00
committed by GitHub
parent 8de7a8b89d
commit 14ed42bd1f
2 changed files with 150 additions and 4 deletions
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25
plugin/hosts/hostsfile.go
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@@ -12,6 +12,7 @@ import (
"io"
"net"
"os"
"strconv"
"strings"
"sync"
"time"
@@ -161,7 +162,7 @@ func (h *Hostsfile) initInline(inline []string) {
return
}
h.inline = h.parse(strings.NewReader(strings.Join(inline, "\n")))
h.inline = h.parseSource(strings.NewReader(strings.Join(inline, "\n")), "Inline hosts entries")
}
// maxFieldSize bounds the memory used while assembling a single field that
@@ -170,13 +171,19 @@ func (h *Hostsfile) initInline(inline []string) {
const maxFieldSize = 1024
// Parse reads the hostsfile and populates the byName and addr maps.
func (h *Hostsfile) parse(r io.Reader) *Map {
return h.parseSource(r, "Hosts file "+strconv.Quote(h.path))
}
// parseSource is parse with an explicit source name, so that entries inlined in
// the Corefile are not reported as coming from the hosts file.
//
// Lines are read with a bufio.Reader and parsed field by field as the data
// arrives, so a line of any length is handled with a fixed amount of memory
// and never aborts the parse of the entries that follow it.
func (h *Hostsfile) parse(r io.Reader) *Map {
func (h *Hostsfile) parseSource(r io.Reader, src string) *Map {
hmap := newMap()
p := lineParser{h: h, hmap: hmap}
p := lineParser{h: h, hmap: hmap, src: src, line: 1}
reader := bufio.NewReader(r)
for {
@@ -200,10 +207,12 @@ func (h *Hostsfile) parse(r io.Reader) *Map {
type lineParser struct {
h *Hostsfile
hmap *Map
src string // where the entries came from, for diagnostics
field []byte // the field being assembled, possibly spanning chunks
oversized bool // the current field exceeded maxFieldSize and is dropped
index int // number of fields already seen on this line
line int // 1-based number of the line being parsed, for diagnostics
comment bool // the rest of this line is a comment
addr net.IP // address of the current line, nil if unusable
family int
@@ -222,6 +231,7 @@ func (p *lineParser) feed(chunk []byte, last bool) {
}
if last {
p.index, p.comment, p.addr = 0, false, nil
p.line++
}
}
@@ -279,6 +289,8 @@ func (p *lineParser) emit() {
if p.index == 1 {
// The first field is the address; without it the line is unusable.
if oversized {
log.Errorf("%s, line %d: address longer than %d bytes, dropping the line",
p.src, p.line, maxFieldSize)
return
}
p.addr = parseIP(string(field))
@@ -292,7 +304,12 @@ func (p *lineParser) emit() {
}
return
}
if p.addr == nil || oversized {
if p.addr == nil {
return
}
if oversized {
log.Errorf("%s, line %d: name longer than %d bytes, dropping the name",
p.src, p.line, maxFieldSize)
return
}
p.addName(string(field))