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Fix handling of truncated legacy errorpage %codes #2411

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rousskov wants to merge 2 commits into from
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Fix handling of truncated legacy errorpage %codes #2411

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4eda297
Fix handling of truncated legacy errorpage %codes
rousskov Apr 22, 2026
9c17b95
Do not dump a NUL character to the debugging stream
rousskov Apr 22, 2026
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22 changes: 19 additions & 3 deletions src/errorpage.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -958,7 +958,8 @@ ErrorState::compileLegacyCode(Build &build)

const auto &building_deny_info_url = build.building_deny_info_url; // a change reduction hack

const auto letter = build.input[1];
Assure(*build.input == '%');
const auto letter = build.input[1]; // may be the terminating NUL

switch (letter) {

Expand Down Expand Up @@ -1261,13 +1262,25 @@ ErrorState::compileLegacyCode(Build &build)
p = "%";
break;

case '\0':
// XXX: Partially duplicates error handling code of the `default:` case.
// TODO: Refactor bypassBuildErrorXXX() to accept `build` and determine the source of the error.

@rousskov rousskov Apr 22, 2026

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I am not addressing this XXX/TODO in this PR to provide a surgical fix that is easier to backport.

if (building_deny_info_url)
bypassBuildErrorXXX("Bare % at the end of deny_info", build.input);
else
bypassBuildErrorXXX("Bare % at the end of error page", build.input);
p = "%";
do_quote = 0;
break;

default:
if (building_deny_info_url)
bypassBuildErrorXXX("Unsupported deny_info %code", build.input);
else if (letter != ';')
bypassBuildErrorXXX("Unsupported error page %code", build.input);
// else too many "font-size: 100%;" template errors to report

Assure(build.input[1]);
mb.append(build.input, 2);
do_quote = 0;

@rousskov rousskov Apr 22, 2026

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We should not disable quoting in this case IMO, but that is a different problem that has a much smaller impact and that is best addressed when refactoring this code to fully address other related problems.

break;
Expand All @@ -1278,7 +1291,8 @@ ErrorState::compileLegacyCode(Build &build)

assert(p);

debugs(4, 3, "%" << letter << " --> '" << p << "'" );
// TODO: Add an I/O manipulator to report non-printable chars better.
debugs(4, 3, "%" << (letter ? letter : '?') << " --> '" << p << "'" );

@rousskov rousskov Apr 22, 2026

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Without this hack, debugging line is truncated at the first % character. It would be better to report the NUL character instead of printing a somewhat misleading ?. This should be done while addressing the above manipulator TODO. I am not addressing this TODO in this PR to provide a surgical fix that is easier to backport.


if (do_quote)
p = html_quote(p);
Expand All @@ -1288,7 +1302,9 @@ ErrorState::compileLegacyCode(Build &build)

// TODO: Optimize by replacing mb with direct build.output usage.
build.output.append(p, strlen(p));
build.input += 2;
++build.input; // skip the parsed % character
if (letter)
++build.input; // when it was present, skip the parsed letter after %
}

void
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